A structural engineer would want to box my ears for this, but honestly...If it's a freebie like that I'd be tempted to just put down a couple of courses of concrete blocks, then mount it on that!
Though I guess a trench, MOT, and C10 is only a weekend's worth with a spade and a mixer.

































If you're not renting, have you looked into a small MVHR unit?
It draws air in and out of the house across a matrix, which heats/cools.
This means that the air gets refreshed, without cooling it too much.