GFRC concrete planters from a melamine and silicone mold

Glass-fiber reinforced concrete lets you cast 1/2" thick walls that would crack with plain concrete. Mold: melamine box for the outside, a smaller box or a silicone liner for the inside, screwed and caulked at the seams. Mix: sand, Portland, water reducer, AR glass fibers, poly fibers, a little acrylic polymer. First a face coat without fibers, then the backer with fibers packed by hand.

Demold after 24 hours, keep damp under plastic for a week (it gets dramatically stronger), seal. A pair of 14" planters costs about $15 in materials and sells for $80 each at a market.