Can't fix the cracks, but for dull scratched up dust covers, get some KIT ScratchOut. This is a very fine plastic polish sold for polishing motorbike helmet face shields, plexiglass, and other plastics. Use a soft cloth and a few minutes of polishing by hand (RESIST the temptation to use a Dremel Tool and cloth wheel). This polish is sold in automotive stores and Walmart. Look for the bright yellow bottle.
Or look for any polish recommended for use on plexiglass and other plastics.