To reinforce some of my styrene pieces, I used a two-part cold cure epoxy & fiberglass cloth. There is no heat generated to warp the piece & the epoxy will not react with the styrene to melt it. The cold cure takes about 24 hours to fully set up & cure so it can be tricky to work with on vertical surfaces. For smaller items like knees, regular epoxy will set up much quicker & can be used in the same way. Just be sure you have enough open time to saturate the cloth piece before the epoxy kicks off. Also remember to sand the styrene pieces well with 80 grit & clean them with some denatured alcohol.