See Circle Device
The most compact to use, it is also the most difficult to construct.
You will need: Lead, or (safer) lead-free pipe solder, container with sand or plaster of Paris, heat source (camp stove, kitchen stove), ladle to melt metal in, large vice and hammer (not needed for lost wax technique).
Cast two flat metal ingots approximately 6 cm x 5 cm x .75 cm, using a sand or plaster mold. Caution: molten metal can be dangerous.
When cool, use hammer and vice to curve them (this is noisy and hard work!). Thoroughly cover in duct tape or Plasti-Grip (the stuff used on tool handles) to avoid lead contact with skin.
Put tape on both sides to create a hinge. Do this while holding the pieces as close as possible to the actual position they'll be in when used, to avoid pinching.
Hold in place with two rubber bands.
Lost wax casting
Alternately, form two wax slabs approximately 6 cm x 5 cm x .75 cm, and curve them to a radius approximately twice that of your flaccid penis. Fill a wide paper cup or other disposable container with enough wet plaster to accommodate both wax forms. Set them on end in the plaster, and let the plaster set for an hour or more. Overnight is best.
Using your heat source, heat the plaster until the wax melts and runs out. (Don't drip the molten wax onto an open flame!!!) You can use hot water to melt the wax, but let the mold dry thoroughly before pouring molten metal!
Pour molten metal into forms.
After a few minutes, drop the mold into cold water. When cool, break away plaster and clean up your cast ingots. File them to remove sharp edges.