Use 1/4 inch plywood or hardwood.
Holding the boomerang like a V, the grain should go left to right, not up or down. Sand the inside of the left part of the V and outside of the right side. Round the rest of the edges.
I tried several versions, v3 worked the best for me.
The first batch I did was only 3-ply plywood, but it caused many of them to break. 7+ply is the most durable, but costs more.