I've looked at those a few times. I've considered using them as a cold frame by only filling 1/2-2/3 full and building a simple top over it. It would save some fill that way. Modified hugelculture seems useful to that end as well.
Discussion
it would proabaly work well. We were thinking of even using one as a dog pen for our little dachshund. on rhis one, I filled it half way with junk wood and dead pepper plants from lsst years season. then topped it off with soil. Still have to get more soil. 8x4x2 can really add up in volume
They don't look like they fill up quick. One of my coworkers sometimes talks about having a little garden and wanting to grow mushrooms. I've told him if he got one of these I'd help him set it up and inoculate some fresh logs to bury in the system.
You beat me to it! Cheers!