Would any of the MTI tubs work? I did a search and below is a link to their solid surface undermountable tubs. (Many of their tubs can be used freestanding or undermounted. Click on the specs link on the product page to see the full dimensions diagram.) We have a freestanding MTI tub that we really like. I know I suggest MTI tubs a lot on here, but we really like ours and not many people have heard of them.

Here is a link that might be useful: MTI solid surface undermountables

I like the MTI tubs but they tend to be quite deep. and wide. I think I have found a Victoria and Albert that will work. It is the Deuville. The company says it can be undermounted. I did not look at it at first because the legs would not go with my bathroom but the dimensions are right and they do without the legs for an enclosed under mount. I'm getting a price and I sent it to my contractor to see what he thinks. Then I can move onto shower fixtures and bath filler.