I haw found a cardboard tub 4 mm thick. And i was thinking to yous "glazing" resin to water prof this. Will this be strong enough to handle this?
If you were to use resin to waterproof the cardboard tube, why not just use the tube as a former, add slip, resin and glass fibres? Or if you just need a cheap res, a pvc pipe and end caps should do it..
If you use the same resin as DIY boat builder use on their plywood boats you should be fine, although I agree with Nexxo a plexi tube would be far easier, unless you're planning a specific look.