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 5: Pack the Clay Into the Handle
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.
Step 12: Drill Pilot Holes
Anyway, make pilot holes for your screws.
Step 13: Mount on the Wall
Hang something up.
I'm not sure how much this can hold, but I'll let you know if it fails.