Step 1: Materials
About spools: I was told that usually a construction company will pay a deposit of $50 USD or more per spool to the cable manufacturer, then return the spool when empty. We were lucky because in our case the manufacturer didn't want their spools back, so they became a nuisance and the foreman put them up on Craigslist for free. But when you see empty ones, never assume they're free and just swipe them, that's weaselly. In many cases you'll find that they are for sale at a reasonable price or for free, but you've got to play nice and ask. Electric companies, phones companies and cable TV providers are all possible sources for spools.
-spool: the one I used was four feet in diameter, and 32 inches tall. The inner "barrel" was two feet across.
-misc. wood: sourced from free palettes, acquired (by asking nicely) from lumber yards and furniture stores.
-galvanized drywall screws
-"door" hardware: hinges, handle, latch.
-hole saw drill bit (two sizes)
-screwdriver (or chisel, if you're one of those "right-tool-for-the-job" snobs)
Step 2: Disassemble Spool
Granted, these are bolts to be reckoned with: 32" long, steel. They sit spaced between fourteen curved slats an inch and a half thick and about six inches wide, all resting in a recessed slot routed into each of the round sides of the spool, to define the inner "barrel."
So all you need to do to take the spool apart is remove the bolts. Sounds simple enough, right? Well, it may very well be, if your spool didn't have too rough a life before it found its way to you. In that case, all you have to do is sit the spool on its "wheels," render either the nut or the bolt head stationary with some vice grips or a wrench (and perhaps a friend to wield it, unless you have an enormous wingspan), and loosen from the other end with your socket wrench. The slats will slide out and fall as the bolts loosen. Then all you have to worry about is crushing your toes or fingers or skull or fine china or small pets when the last bolt gives way and the heavy round chunks succumb to gravity. But if only a little rust is present, plan on using a great deal more profanity and other rust-battling measures.
Step 3: Make New Bolt Holes
I figured out where to drill by drawing lines from the center point of the round, across an existing bolt hole, and out near the edge.
I cut a shallow hole using a 2" hole saw, then a deeper one with the same center point using a 1-1/2" hole saw.
The wood between the hole saw cuts was easily chiseled away (I used a screwdriver, not a chisel - don't tell, okay?) until the washer had a comfortable place to sit.
After the recess was roughly shaped to the contours of the washer, I drilled the hole for the bolt itself. Note: It's important to do this after you use the hole saw bits, because a hole saw needs wood to "bite," and would slip around dangerously if you were to drill the bolt hole first.
I repeated this for all six bolts on both sides of the spool, for a total of twelve new holes.
Step 4: Loosely Assemble
First, my measurements weren't perfect, so some of the bolts were a tiny bit misaligned. Not a big deal: that's why they invented hammers.
Second, the bolts were, in fact, long enough to work even when not sitting in slots. See, in moving the slats to the outside, I was increasing the overall height by about an inch, because I didn't route out slots for them in their new positions out at the edge. Luckily there's enough play in the threading and length of the bolts that I didn't need to route a slot to make it all fit, but that might be worth doing if you have the tools and the inclination.
Careful not to smash my toes or fingers or skull or fine china or small pets, and happy that I wouldn't have to go back and deepen all of the bolt hole recesses, I dropped the cumbersome assembly down onto its "bottom" for the next steps.
Step 5: Add the Boards
Various (mostly 1"x4") slats harvested from palettes filled in the remaining gaps.
Step 6: The Door
I started by tracing the arc onto some boards, and sticking them together to make an assembly reminiscent of a rocking horse, that rocked rather well and made me kind of want to make a rocking horse. But I didn't.
I temporarily hung the rocking-horse-looking frame into place, accidentally putting the hinge on the right side when I meant to make the door open the other way.
I took it off and added more slats, so that it started to look like a medieval shield of some kind, and made me kind of want to make a medieval shield. But I didn't.
In retrospect, I should not have maintained a nice arc; I should have flattened the arc where each slat touches it. That would have made attaching them much easier. You really should do that if you make one of these.
I hung the door, and added a handle and a latch.
Step 7: Fill & Paint
Since this house will be placed in a large aviary-like enclosure (see photo) safe from raccoons and other predatory beasties, and since ducks don't just sleep all night like chickens, the duck entrance didn't need a door. So it didn't get one.
It did get a ramp, though - made from palette wood, and just sort of parked up against the duck door.
And that's it!
The ducks love it, and so does the occasional cat.
Update: the duck house just got a new addition: a bench/cabinet that sits on top. See it here: