Use tape in a "T" shape for a handle on the back side, Gorilla tape works very well. It lets the blocks flex while giving you something to hold the block with.
Here are a few I've made, I have multiples of the same size blocks in different thicknesses. The angled-end block is helpful around raised fenderwell openings or similar stampings.
I use smaller and thinner blocks without tape for wetsanding clear. The hard surface cuts the orange peel out very effectively but they're still flexible enough to sand curved areas. The edges are rounded over so they can't dig in.
