I'm a fan of the inexpensive, easy, "gravel box" which is literally a frame made of 2x4 or 2x6 that you fill in with 3/4" to 1" crushed rock. The box should be sized so that the hot tub sits on the rock only and is uniformly supported. I like that size of rock because it reaches 85% compaction under its own weight. Smaller sized rock can be sort of spongy.