Mold for Concrete Birdhouse (with Secret Compartment)

I’m not much of a bird watcher, but I do like the idea of a secret stash disguised as a birdhouse. I’ve designed a mold to cast such a birdhouse in concrete. Plastic doesn’t like UV light, but concrete handles it and weather very well. About 2/3 of the space inside this design is birdhouse, but there is a small, triangular sealed compartment in one corner (4” base, 2” high, 5” deep). It cannot be accessed once the birdhouse is made, so it is ideal for storing something semi-permanently (years or decades). But for those of you who don’t need this feature, I’ve also included files with a hole for birds to access this compartment. You can make a boring old birdhouse if you wish.
Overall dimensions are about 10” tall by 6” wide and 5” deep. I wouldn’t recommend scaling down, since that will make the walls too thin. The design takes about half a kilogram of filament for a single mold, but since the molds are single-use, color and strength don’t matter. It’s a good project to use up remains of spools from other projects, as well as miscellaneous wire bits and concrete.

Materials Needed:
• Approximately 450 g of filament. These are single-use molds that you won’t have to look at for long, so use the cheapest, ugliest filament you can find or happen to have on hand.
• 10-15 lbs. of concrete mix.

• Wire for reinforcement and hanging loops. 1/8” diameter wire or the like should do fine. Several lengths at least 5” (13 cm) will be needed.
• Soldering iron/woodburner. This will be needed to make and seal some holes in one of the molds.
• 18 M4 bolts (16 mm or longer) and nuts with which to bolt the molds together. M3’s or 4-40 screws would also work. Not all of them are strictly needed and not all of them need to be this long, so if you’re a few bolts short, no problem.
• Petroleum jelly as a mold release agent.
• Pliers for bending and trimming wire.
• Optional: Sandpaper to smooth the molds, paint/stain, concrete dye.

Printing and fabrication instructions are in the included guide document.