Waterproof material suggestion

I have to build a robot to function underwater for a university class. We have the rest of the semester to build it so it won’t be a simple machine (it is meant to clean the floor of a water tank). Assuming the robot is around 1’x1’x1′ and I have to build the casing from scratch (so I can’t just take a plastic box and stick robot parts in it). What’s a good, cheap (~$100) material I can use to be build the casing?

Any help is appreciated since I’m still pretty new to robotics :/

submitted by /u/LeanMeanMcQueen
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