Ply wood. I used resin and some mat to bond the two. Lay them flat on the drive or in the garage, and the screw them together. Put something under them to catch the extra resin. Take a 1/2" rounding over bit in a router to take that 90 off it, glass will roll over it better. Get some cabasil to set the stringers in with (fine powder mixed with resin, make a thick paste). 1.5 oz mat then put some 18 oz bi directional over that, better to do it all at once. Then if you want cover the whole thing with some cloth to make it perty, yes perty. Piece of cake. Make sure you grind all the surface area that will be getting the new glass, like 24 or 36 grit. That is not a fun looking task you have ahead of you. Oh yea when done spray with gel and webbing solution, even pertier, yes pertier.