To be honest, I can't think of a single tub I've seen that is a good enough size for even an IJ.

A 5 foot IJ needs at least 3x2x1, preferably 3x2x2.

The other subspecies available in the US (diamonds, jungles, and coastals) all get big enough to need at least a 4x2x2. They really do use the space. My coastal girl at a year and a half old is already around 50''+ and makes full use of her 4x2x2. She's not very heavy though, just really long, I'm sure she's still under 700 grams when empty.

IF you can find a tub with big enough dimensions, there is absolutely no reason it would not work for a carpet python. I just haven't seen any, and I just adore my Animal Plastics cage because it is so nice (but as you stated, so expensive).

I'd suggest you check out www.moreliapythons.com, they were a great resource for me when I was looking at carpet pythons, and they even have individual forums for the individual subspecies.