So, being an inventive jackass of all trades, I went ahead and found out how to go about making one on my own, and it turned out to be pretty damn cheap!
This is my first DIY Instructable, so I apologize in advance if it seems as though I have no idea what I'm doing, simply because I may not. However, I digress. Onto supplies.
Materials you'll need:
-a potato masher like the one in the picture
-Dremel with metal cutting blades or a hacksaw
-red (not sure of the strength rating) physical therapy band, or one thinner and stronger (yes I know the band in my picture isn't red, I started with a weaker band that performed like crap)
-piece of strong, supple leather about 2" x 3"
-zipties or wire with a strong breaking point
-knife or scissors
-either a chainsaw sharpening dremel bit, or steel files
Step 1: From Masher to Slingshot Fork With 2 Cuts
It is crucial to the longevity of your sling bands that you round off your cuts as smooth as possible, with no barbs left over that could later cut into your sling bands as you draw them back and put tension on them.
Step 2: Cutting Your Ammo Pouch
...no, not THAT kind of fun with leather, that's another site that takes your credit card info before you see the good stuff. Its all free here! Anyways...
Take that scrap of leather I told you that you would need in the intro and... WHAT? You don't have it yet? Fine, we'll all wait for to get your bit of leather and come back.
. . .
...are we all set? Good. Take that 2" x 3" rectangle of leather and cut it into a crude diamond pattern, like so. This shape will give you the deepest pocket for shot once you shoot this thing for a few weeks and the leather starts to stretch out in the middle.
Punch some holes a little bit smaller than the outside diameter of your tubing at each end of your pouch about 3/8" from the edge of the leather. This will give you a bit of leather to fold over itself to strengthen the pouch where you'll tie your tubing.
Step 3: Tying Your Bands to the Ammo Pouch
Now for tying your tubing to the pouch, a friend with a killer grip and a slender but strong steel rod will help in holding the crucial fold in the tubing used to start a square knot (if you don't know what that is, find the nearest Boy Scout Troop and ask to borrow one of their handbooks for a minute or two).
Wrestle the knot closed once its properly centered along the side of the leather pouch, and tighen it as much as human strength will allow after getting that steel rod outta there. Repeat for the other side, and we'll move on to final contruction step.
Step 4: Securing Slingbands and Fork
Well now it is time to feed each free end of your slingband onto and past those right angles on your fork, until each side has 6 or 7 inches of free-hanging band dangling from those right angles.
To secure the bands to your masher fork, either really tighten down a pair of zipties or a strong, twistable wire around the bands and rods of the fork just under the right angle bends. Doing this twice will add redundancy to your anchoring system, and thus reduce the likelihood of the bands slipping backward at all.
If all of this wordiness is confusing, just look at the pretty picture below to see what to do here.
Step 5: Go Have Fun With It!
Now grab some steel ball bearings or lead shot, and let fly some missiles from your new slingshot! I hope you enjoyed my first Instructable, and feel obligated to post some HELPFUL comments on my intro page.