I got some 1/16 in black textured abs from a local plastics store. Made a pattern out of poster board and cut it with tin snips. Cost me $12.00 and looks good.
If I were doing it again I would get a piece big enough to go from the window frame down to the floor level.

1949 chev 3100 3 window. 350hp/327/m21 4 spd, 12 bolt rear, power brakes, 3" dropped axle from Sid's. Disc brakes next.