I think that the shower unit idea would be prohibitively expensive, simply because the tubs are made to higher quality than what you need for tree frogs, and you would ultimately pay for that.
I think your best bet is to go with a custom enclosure, preferably made with glass walls and a bulkhead drain at the bottom. Sure this would be expensive, but I figured that you wouldn't consider converting an entire room without planning on spending a lot of money. I don't think that the whole room idea is very practical. Plus I always try to emphasize to people that living situations change and five years from now you may need part of that room for something else.