Have two racks, 10 total tubs based on mixing tubs similar to what you describe. Never had a problem with chewing at that hole, too far to reach from the inside. Of course like all rodent tubs, tight fitting is a must.
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Thanks,

Dave Colling

www.rainbows-r-us-reptiles.com

0.1 Wife (WC and still very fiesty)
0.2 kids (CBB, a big part of our selective breeding program)

LOL, to many snakes to list, last count (05/26/2009):
36.51 BRB
29.42 BCI
And those are only the breeders

lots.lots.lots feeder mice and rats