
LOL Yes it would need a lot of hard work, but you would lay a nice thick layer of gravel, shingle or something like that as a soakaway in the bottom of your hole. You put the bath on top of that, it wouldn't need to be sunk fully, have it sticking out as much as you want - just pull out the plug to empty it. It wouldn't need to have the water changed all the time, but scoop the leaves and scummy bits out!
Couldn't really do it if you had children around though so it would have to be a large plastic pool :) like the sandpit ones :)