Eps
XPS
Your call. Both are polystyrene and will dissolve. If you do it, make sure to document it and share. Also, make sure to do some thorough testing before slapping it in your ski. It might be a fun project, or it might be a complete waste of time. I've thought about building one, but quickly came to the conclusion that there are better materials to use for this purpose (hdpe, aluminum, etc). I think with your future in engineering, you should spend the time modeling a tank in solidworks and analyzing the cost to have them injection molded. You may never make a tank, but the experience will be far more valuable than the headache you will get from trying to build a composite tank with the materials and processes you have available to you. Just my 0.02, take it for what it's worth.
Or just forget he whole thing and go work on your backflip.
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