I'd suggest sanding or sandblasting the back side of the Lexan.
Conventional indicators use parabolic reflectors or Fresnel lenses (or both) with omni-directional incandescent bulbs to get most of the light straight out the back to be visible to following traffic, with some dispersing up and to the sides so you can see the lights from all angles. That won't work with pre-mounted LED strips.
If you sand the back of the Lexan, the directional light will be diffused and spread out all directions more or less evenly. Standing near the truck it will seem bright, but it might not be bright enough from far away to the rear. Wouldn't want you to get a ticket for insufficient lighting.
Before modifying anything, have you sat in another car from say 50-100 feet away? Maybe the existing lights are hard to see while standing next to the truck but bright from road level.
If the lights are pointed downward due to the curvature of the roll pan, and you can't make some kind of wedge to point the LED strips straight back (or the lamps would look bad if you did) you probably don't have much choice other than to try diffusing the light in this (or some other) way.