Lowes carried Plexi and if you take the measurements with you they will cut it to your specs.
Your going to want it close to snug for fit then just silicone it them into place.

Use something pretty thick and not the thin stiff since the thinner the more it will flex.

If no Lowes, check a local auto glass place and see what they would charge. If you take the tank, they will probably measure and fit for you.