Truck Cap Side Windows on the cheap

The best thermoplastic sheet for this application would be cast acrylic. Regular plexiglass is extruded acrylic. Cast acrylic is stronger, more scratch resistant and has better optical qualities. The acrylic sheets come in a variety of colors, tints and mirrored finishes. Standard nominal sizes are 1/8", 3/16" and 1/4" thick. 1/8" is weak. The OP would have been better off with 3/16" or 1/4", much stronger. If you buy a standard size acrylic sheet(mostly 4' x 8') from a plastic distributor, you'll pay a lot less and have better selection than buying pre-cut pieces at a retail store.
A dark tint acrylic would be about the same as tinted glass. Mirror finish would reflect heat and provide total privacy, but I don't know about having giant mirrors on the side of the truck.