[Feroz1325]

1/4 inch lexan will do the trick, however 1/4 plywood will be more that adequate to keep the frame stiff. We did that last year in our sheetmetal chassis and it worked great! If you have access to a waterjet/plasmacutter/laser you may want to consider a pocketted 1/8 (or thinner) aluminum bellypan....keep time in mind as it is already week 6.
Good luck from team 1325!