I have worked with West Systems epoxy. It is pretty easy to work with. You can buy pumps that fit on the cans that measure out the ratios for you. That makes things much easier. I have yet to try to make something that I use to hold water in but, based on working with it I don't think it would be hard to do. You do need to let it finish curing for a week or two to ensure it is done giving off fumes.

I am not sure about big blocks of polystyrene but, you can get large sheets of it at places lie home depot for home insulation. You can cut it and glue layers together to get greater thickness.
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Steve