Take it that you have used the Lighting Gels on a model before, what is their thickness?
if you have a theater or movie lighting supply store you can get "gels" that they use to color their lights for stage work.
For ease of workability 0.010 ( 1/4" = 0.250 so 0.250 / 25 = 0.010 ) or 0.015 ( 0.015 X 25 = 0.375 or 3/8" thick ) both are stock sizes for plexiglass and would be ideal. Measured the clear sheet in a K. S. Pittman Willys kit and it is 0.006 thick. ( 0.006 X 25 = 0.150 or 5/32" thick ) might be a little thin for a race car. Oops, sorry the Engineer in me pops out every now and then!