Save the digging!!!!
For several reasons.
One is when you wish to drain it, it is all hard yakker.
When you want to service it, it is all bend over.
I put several baths into the ground here so I know ewhere I'm coming from.
The last bath I did, I built a cradle and used polystyrene for insulation and used corrugated iron for the outer skin.
Looks ok, and if you want to move, you don't have to find fill to fill in those holes left by the in-ground baths.