Drainage is important, and you know that. I would use a water shield as an outer barrier. 2" poly isocyanulate inside that against 4" cinder/concrete block. I used 1/2" radiant tubing inside that and former the inside with mortar. These were mud bath tubs at a spa. 30yrs later they are still going. I am not sure what you would want to use for an internal finish. It would be nice to have a smooth finish. Seems it wouldn't matter in a mud tub, but would in a bathtub. Also, I guess it being outside you will need a cover as well as a cap for the walls.

Good luck Drake. There is no inground custom bathtub outa concrete industry, manual or design tome. Just get in there and build what you think best.

As long as at least one tub remains in the house, removing one shouldn't be a problem. Under this scenario, a tub would be in an auxiliary bath that is commonly used or available for use by children. In this case, no tub would be necessary in a separate master bath. If you had 3 or more baths in a house, at least one should have a bath. The bath versus shower controversy has been going on for about 50 years. While there are always exceptions, more and more adults have become shower only people. Adult bathers are a much smaller community and getting smaller everyday. Ideally, I would keep at least one bathtub in a home. By having a bathtub a potential buyer won't exclude your home when considering a purchase.

To all responders - many thanks for the comments and suggestions. Was able to open access door at plumbing end of tub and was able to see steel mesh and clumps of plaster below tub rim on exterior wall but no sign of studs - so I assume that plaster with mesh support is also behind wall tiles. This would explain erratic readings with stud finder. Exterior wall is brick - not sure if stud finder will give reliable readings through brick but will give it a try. If so will give me some general idea where studs might be.