I used poster board to make a template. Then I bought the Lexan sheet at Lowes. For free Lowes will cut the sheet into squares that matched my dimensions. For the top piece that has curved corners, you just extend your measure as if the sides and top edges meet at square. I bought the $5 scoring tool. You lay your cardboard template on the top piece and use a felt tip marker to trace the left and right curves. Next just take your time and score along the trace curve. When you have a deep score, take pair of pliers and break off to get your custom curve. Sand for smothness if it feels rough.
2016 Wildcat 295RSX - 2015 GMC 2500HD DblCab Duramax/Allison 4x4 Z71 6.6' Bed
Maxxis 16" E; Ride Rite AirBags w/AirLift 1 wireless compr; Adv.Folding 4 Cover
Andersen Ultimate II Aluminum 5erhitch on Reese Flipball w/Curt 4" offset ball
LCI Ground Control 3.0 4pt + 2pt Elec.Rear stabilizers; 3 Slidecovers; 10ft. Portabote
6000# Dexters; mor/Ryde CRE3000 w/HD Wetbolts; King Dish Tailgater; 5hp Outboard
Dual A/C; Furrion 48 Rear Camera; Champion 3100 Inverter w/Extended Run