The trick I think is it needs to be insulated on the outside of the tub. The interior of the tub could be concrete , then insulate the exterior and underneath with rigid foam. If you insulate the interior there is no way to bond anything. Unless you where to do a layer of wire mesh on the inside and skim coat that with a lightweight concrete, then the newer fiberglass mesh and bond. Just spitballin here...
Well mudmixer, you are a concrete guy, how would you build one if a customer asked? This question is for all those concrete guys actually.
Me , I would use an old clawfoot bathtub, outside, and fill it with warm water when I need to relax. Then just pull the plug when your done.....and wrap up that frozen hose you just used to fill it.