A big pit works fine if you remove the legs from a standard trampoline. The downside is that you do not feel like you are bouncing as high, because you don't get to start with the extra 4 feet in elevation.

We just dug a pit. it needs to be deeper than you think though (or well drained) or you get wet feet in the winter!

It was a trampoline designed for this type of install. The frame bolted onto a retaining wall type rectangle I built.