Sealing my enclosure

[I Love Dino]

I built Dino a 5'X3'X8" plywood enclosure yesterday and my dad thinks it's going to rot in a couple of years so what can use to seal the bottom so that it's water proof. I'm hoping it can last at least 15 years. Thank you!

 

[yagyujubei]

Heavy plastic, linoleum, cheap floor tiles, visqueen, old shower curtain, etc...

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scrap rubber roofing