Bass equipment lists dimensions as 34 1/2 X 22 1/2" and that's more exact than I get when sizing liners. We don't sew ours and just fold it over. All it takes is finding a bolt of fleece that matches a dimension of the pan or is twice a dimension and cut the other side to match. It doesn't have to be exact. Right now most of ours are 58-59" depending on brand we bought and we cut it to 1 yard sections (36"). Then fold it in half and any extra edge into or around the pan.
If you want 3 layers cut it 3x one direction and fold it twice. Makes it pretty easy and you can weigh the corners down with things if you don't cut it long enough to wrap the pan. Most of the time we use clean bricks or sometimes flooring tiles in the corners or down 2 edges.
Even for sewing it you certainly don't have to be exact. Chinchillas do not care about ridges or folds in their fleece and will pull and shove them out of place within a couple hours.