I have a rowing machine (a WaterRower) with a water tank (picture). The tank has a few cracks on the top. I'm thinking of gluing a piece of plexiglass on it as a patch but was wondering if anyone had a better idea.
Add'l info: the tank is 23" in diameter and 9" tall. The plastic is ~3.5mm thick. The cracks are roughly 2" long. I'm not sure what caused them. The tank is normally sealed except for a bung hole in the top (visible in the 2nd photo) but it's currently in 2 pieces so I can work on it. I can't tell what the plastic is made of (no markings) but the tank has a little give when pressed. Here are 2 photos taken from the inside: 
