Hmm, turning a tub into a display cage?

[The Serpent Merchant]

I'm also wondering what I would use to adhere the glass to the plastic..
Would plexiglass work or would it be too flimsy?

I would go with ⅛ or ¼ inch thick plexigalss and rivet it to the plastic. There are adhesives designed for this as well. Even machine screws and acorn nuts would work.