Chewing trouble

Anyone have trouble with their rodents chewing tubs? I originally had 1.5 rats in each tub and they started chewing after about a month and a half after I got them. So I figured it meant they were over-crowded. So I took out a couple and put them in another tub. But they still chew. The holes are big enough, that the largest rat (300+ grams) can stick her entire head through. I have the tubs that are recommended on this site http://www.arbreptiles.com/cages/rat_rack/ratrack.shtml

Anyone got any ideas on how to make them stop?

Yea mine do that its because there is a ridge or a little dip that they can get their mouth on. Also there is one doing it that rat will teach the rest to chew out.

When I decided to build my own rack I opted not to use the concrete mix tubs thinking they would be too easy for the rats to chew through, instead I built a rack with jumbo kitty litter pans, never had any problem.