a 6-7ft carpet of any subspecies would need at the VERY minimum a 3x2x1, preferably with more height. I still think that is too small in the LxW dimensions, but it can be done.
An 8ft carpet would need a 4x2x15'', bigger is better.
If you can find tubs in those dimensions or bigger, there is no reason they wouldn't work. I just don't know of any tubs that size.
My female 5ft girl is in a 4x2x2 with a basking shelf right now and makes full use of every inch of it. I might need to get something even bigger for her if she gets bigger than 8 ft.