I've gone through all sorts of imaginable substrates for my racks and I came up with a couple that I thought were of mention.
First off, in Six Qt Sterlites some people use for hatchlings. I've found that Bounty Select-A-Size are a perfect fit, if you like paper towels. I've used one or two (folded over) in the past with great results. They're pre-perforated so they fit wihtout any cutting or folding edges. For the larger 11x16 size, I've found the Kirkland brand paper towels are the right length, from perforation to perforation, but need to have about 2" folded lengthwise to fit.
The 11x16 size (can't remember how many qts) ones are a little harder to find a perfect size for, but I came up with this idea. This is a roll of masking paper for paining that I found in the paint section of Home Depot. It runs less than $2 a roll, which is 180 feet long. The 9" wide size works great in this tub and the 6" size fits great in the 6qt mentioned above. It takes hardly any time at all to repaper my bins when I'm cleaning them now. This masking paper isn't as absorbant as paper towels, but it doesn't bother me. I find it's about the same as newspaper, but quick and cheap.
Anyway, I just wanted to let others know what I've found works pretty well.