I have made more of those than I can count. It's a great and economical way to get a kid into a bow. Most of the time involved is waiting for the glue to dry.
If you pick the slats carefully, there is no need to back them. That said, I have backed quite a few of them just because my grandkids all have cute little faces and I want them to stay that way!
I have backed them with fiberglass Auto body mat and epoxy, cotton duck and wood glue, rawhide, and brown paper shopping bags. None of these backings improved performance. In fact, they hurt performance due to the fact that they do no work but add weight to the limbs. They all work and the paper bag is the lightest, easiest and most economical so that is what I have settled on.
A pyramid is the best design for the material. Bring the limbs to points at the tips, the narrower the better. Gluet a piece of scrap from the limbs over the tips and file a string groove across the top of the overlay, but not into the side of the limb.
That's the way I settled on doing these and it works fine. There are some pics somewhere and I'll post some if I find them.