Gascort,
If you have cracks in the plexi, drill a hole (with a dullish drill*) just at the end of the crack then fill the hole with silicone. This is a stress relief technique, that spreads the forces around the circumference of the hole. Stops cracks in their tracks...
*plastic drills should be ground so the cutting edge has a 90 degree rake. You want to scrape the plastic. A normal twist drill will dig in and force the drill through too fast, causing fractures. Barring a way to regrind, use a dullish one, go slow and watch out for overheating. A glass of water can be helpful to cool off the drill if you are doing a lot.