Step 1: Things you need
Small soda bottle cap
drill for hanging
drill bit for a pilot hole
three longish screws
polymer clay (or something else to fill the inside of it for strength)
some commenters suggested using concrete instead - which is inexpensive and an interesting alternative
vapour mask and hot knife (or you can use scissors though I find the hot knife easier to control)
largish glass or plastic container
(optional) plastic paint
Step 2: Cut away the handle from the bottle
Step 3: Condition the clay filler
Step 4: Roll the clay into a long tube
Step 6: Smoosh the cap on
Pack it in there.
Step 7: Pack the clay into the triangular base
(Try to do this with all one piece of clay.)
Aim for a very flat even surface.
Step 8: Trim away excess clay and plastic
Step 9: Submerge in water
Step 10: Microwave for ten minutes
Credit where it's due: I searched online and found this website where the author, Garie Sim, documents his successful experiments with microwaving polymer clay. So Kudos to him for this great technique.
Step 11: Allow to cool
After it is cool. Check with a fingernail and make sure the clay is not pliable.