doubt you'll find anything ready made those dimensions, but there's no reason 6 mil poly won't work fine, I've used it for tanks a number of times - if you're worried about poking holes get 4 year greenhouse poly, virtually impossible to tear it, should be available at any ag supply - or if you know any farmers that do silage, get hold of some silage bag film, it really is indestructible, would be fine in a trough made of heavy cardboard
Bill
