I made a flat longbow for my 3 1/2 year-old grandson with just two thicknesses of walnut. Total thickness is 3/16" and it pulls about 8 pounds. It had only one thickness last year. I'm thinking of adding a strip of glass on the face for next year. So-o-o-o-o to answer your question, I guess I'd get the thinnest glass I could and you'll still probably need to make very narrow limbs to hold the draw weight down enough. Not really the kind of answer you were looking for, but that's the best I can do.