I am going to be building a couple curved poured concrete landscape walls and am looking for some advice on form material and construction. The walls will be 3' tall and 6" inches thick poured on a 1'X6" footer. One will be a constant curve, the other multi-curved, and they'll be cutting the corners of an existing block wall to be used as raised planters. The tightest radius of any of the curves will be approx 2'. I've looked some at using curfed plywood or using regular 5/16" plywood curved across the grain and doubled as it should be able to bend to that radius. Any advice on the easiest and most economical materials/ methods that will reliably hold for the project would be appriciated.