Tripod-Head to Monopod-Head Adapter on 43 Cents. Literally.

Short version of my story:
I bought a camera, it came with a bundle of accessories, including a Samsonite 1100 tripod. I have a monopod. I want to go take photos with a swivel-head on the monopod really soon, and didn't have 40$ to spend to get one locally.

I need an adapter, and I need it now. (But I'm putting in an online order for a "real" monopod head anyhow.)

Lo, and behold: a bolt, half a rod coupler and 43 cents later (it was 5am and I decided that it would be awesome to see just how jury-rigged I could make this thing), I have a little mount that will let me use the swivel-head on my monopod. I don't exactly trust it, but since I plan on getting a "real" swivel head, this'll do in a pinch.

These steps are for what I did, specifically. You may have to make alterations, and I will try to note where other models may require different steps/sizes/consideration.

Materials:
1/4" x 2" bolt
Threaded rod coupler/ 2 nuts (either item may be referred to as a "nut" here)
1 quarter (or 1/4" fender washer)
2 nickels (Mandatory for Samsonite 1100)
8 pennies (or 1/4" normal washers)

Tools:
Hacksaw
Drill (+ metal drilling bit)
Clamps will make your life easier
Epoxy
Calipers or crescent wrench (for greater ease in comparing diameters)

Basic idea: the swivel-head comes off the tripod, but the clamp needs something the right diameter to hold on to, and that thing needs to attach to the monopod. Nickels are exactly the right diameter for me. The quarter acts as top cap, pennies as the spacers and the nickels as the only real contact points this widget makes with the clamp. The bolt is epoxied to the nut, and one spacer, but all other items are free-spinning.

Disclaimers: I'm sure this voids several warranties, but I'm not responsible. Don't saw off an arm. Et cetera.

Step 1: Disassembly & Measuring

Disassemble: Take apart your tripod. Unscrew the various screws, and such--it's pretty self evident what you need to do. There might be a little cap or sticker over certain bolts, but you can see what you'd need to pry off in most cases.

Measure and match diameters: Locate the point where the tripod legs are linked to the head. Think of it as a ball and socket, except in 2 dimensions instead of 3 ("cylinder and sleeve" is more accurate, I guess, but it's not as effective as a descriptor). Examine the diameter of this joint. Locate plastic or metal discs or cylinders of equal diameter. The US nickel works for my Samsonite 1100.

Guesstimate spacing: Find "spacers" to stick between, above and below your nickels. Having the nickels stacked right next to each other will make the head wobblier (if you stood with your feet close together, you'd be easier to knock over than if you stood with your feet slightly further apart), so find a way to distance them.

You will want enough spacers to accommodate the height of the sleeve, as well as any part of the whole head assembly that might jut into your monopod. Prepare a few extra spacers, just in case.

Note, however, the Samsonite 1100's bolt that tightens the sleeve sticks out such that the inner diameter is not consistent on the sleeve. You will need to make sure your nickels are not going to get in the way. Right now, it's hard to plan for this. I find it easier to prepare the entire adapter and then fiddle with the spacers right before I epoxy things in place.

Step 2: Pick Your Poison (or Coins)

Some coins are more equal than others: Wear & tear will factor in on which coins you use. I am using nickels and pennies in the sleeve, and a quarter on the cap. New nickels provided an overly snug fit in the sleeves, but slightly worn nickels worked very well. Pick an older, worn looking nickel. Try a few out. As much as I'd like to put holes through the ugly new nickels, it's more effective to use an old one. Monticello on the reverse will also help you drill centered holes!
Pennies tend to wear noticeably thin at the edges, so newer pennies will be more even. Pick newer, shinier pennies. The Lincoln Memorial will help center holes.
Quarters with an eagle on the reverse makes centering holes easier.

In short: pick coins with symmetrical images on at least one side.
Pick worn nickels.
Pick mint pennies.

Step 3: Deface National Currency. or Use Washers.

Drill lots of holes: if you're not using washers, put holes through your spacers & nickels. You want the holes to be marginally larger than the threading you need to accomodate on your monopod. My threading was 1/4" and so I used my 1/4" drill bit and then wriggled the bit around once I'd drilled through.

Where to drill: If you followed my suggestions and have coins with buildings/eagles on the reverse, here's what I've found works.
On not-so-worn nickels, you might still see the architectural detail on the pediment (it's the triangle over the doorway). Center is approximately between the pediment and the top of the doorway. (But see below for drilling nickels off center)
On mint pennies, you can probably see Lincoln in the memorial. Nail him through the stomach. It probably doesn't matter much if you're off center on the pennies. As long as none stick out past the nickels when on the axle, you should be fine.
On the eagle quarter: er. uhm. uhhh. Crotch.

Hints & tips & Learn-From-My-Mistakes: (But you are quite welcome to make your own. And share! I might add to this list.)
-Mark the center of your coins and make a punch mark for easier drilling.
-Unless you've got some awesome ideas about clamping coins up/down/left/right/sideways, it's easier to drill them individually rather than as a stack. (The bit will tend to catch and then spin the coins rather than actually going through them.)
-Clamp the coin on some wood so you can drill through fully.
-I found that some coins will deform is enough clamping pressure is put unevenly on the edge, so I usually place a second coin beside the first and have the clamp hold both down.
-It might be better to have your nickels drilled slightly off center(!) If you tighten the sleeve around two centered nickels, the nickels will continue spinning on a center axle. If they're slightly skewed, the clamping might be enough to have them lock in place on the axle, assuming the holes are close enough to axle size. Drill a couple and play around during final pre-assembly.

Step 4: Start Stacking

Apply common sense/logic and try out some combinations of spacers & nickels: You should have a pile of washer-looking things. Or washers. Grab a bolt that's slightly longer than the sleeve height + whatever else you need to keep clear of the monopod.
Stick your quarter on, and then at least one spacer before you put on a nickel--this will help make sure the nickel is held by the sleeve rather than hanging out at the top. Put a few more spacers in.

Tightening-bolt & nickel conflict: Stick the bolt + coins in the sleeve and see where the tightening-bolt crosses the stack, and make sure you don't have a nickel going there. My tightening-bolt is pretty close to center, so I chose to have one nickel above and one below the bolt. This makes the adapter a little more stable, but it also forces me to remove the bolt to swap the contraption in and out. (But I need to remove the bolt to get the head on/off the tripod anyhow.)
You may choose to have both nickels above the bolt such that putting the adapter in won't require removal of the bolt.

Your entire stack need not be the full height of the sleeve. Putting the nuts on that will attach to the monopod threading will also take up some space on the bolt. Figure approximately two nuts' height plus your coin stack needs to hold the head high enough off the tripod to prevent parts from jamming. Add a little clearance to allow the coins to spin--one to two millimeters should suffice.

Size the bolt: Guesstimate the distance the monopod threaded rod will extend into the nuts. You should probably leave at least this much clearance on the nuts, but the rest should be occupied by the bolt that is acting as your adapter's axle. Figure out how short you need to cut the bolt so that it is about half-a-nut-height longer than your stack of coins. (Make sure you are measuring on the bolt length and not including the height of the bolt-head.)
Remove your coins, cut your bolt, and see how well you calculated: re-stack the coins on, screw on bolt #1 and see how much space is left to accommodate the monopod.

Step 5: Epoxy Stuff

Final dress rehearsal: Stack things on your cut bolt. Put one nut on the bolt, put the other on your monopod. Stick the adapter in your swivel-head, match the two nuts up and see if you got your measurements right.

If yes, then you are ready to epoxy stuff together. If not, go back and fiddle with the stacking, cut a new bolt. Something. Troubleshoot it yourself.

You will want the nuts epoxied to the axle-bolt. I chose to have one spacer also epoxied to the nuts, but that was more to guarantee the other spacers wouldn't get epoxied together accidentally. I used washers to prop the final spacer up against the nuts so that epoxy getting in between the bolt and the spacer wouldn't seep further down.

However, you may wish to do the epoxying in multiple steps: The epoxy didn't hold on my nuts and the pair came apart, so here's what might work better.
First epoxy both nuts together (if you chose to use a threaded rod coupler, you can skip this alternate sequence) so you can see how well the assembly works before you irrevocably glue things together. You can always glue two more nuts together if your first set is the wrong length.
Thread both nuts onto a bolt (possibly a scrap bolt, in case you're overzealous and epoxy too much) and epoxy them together at the seam. Let set. Take 2xNut and gently test your adapter out. If things seem to be the right size and all, epoxy the 2xNut to a spacer and put a collar around the nut. This should be slightly smaller than the spacer diameter. You will fill the collar with epoxy to create a solid wall around your nuts. This will help prevent the nuts from tearing apart. Epoxy putty might also come in handy here, if you don't want to mess with filling a collar with liquid epoxy.

Or you can do what I did after my first epoxy job came undone, and use a threaded rod coupler trimmed down to be about the same height as two nuts. (Yeah, I cheated. But these are my rules, so neener.)

Step 6: Try It Out

Put the adapter in the sleeve, put the swivel-head on the monopod and see how it works.

& congratulate yourself on a job well done.
Or congratulate yourself on making it this far through a wordy instructable.