It looks like the U-Bars pass through a panel of reinforced vinyl that attaches to the bottom of the pool to form the triangle. It doesn't look like you need a reinforcement under it unless it is very soft ground. If so, I'd suggest pavers, but you'll want to dig it in so it's level with the ground and the vinyl panel lays flat on it. Pavers generally look nicer than pressure treated wood. The roughly 8"x16" pavers should do nicely, one under each U. They come in red or grey. You COULD do the same with 16" lengths of 2"x8" pressure treated--or cut them as you like.

Just my thought..