Hi Tim,
Realistically, my suggestion would be to fill it with manure and put 4 or 5 inches of leaves on top and let it sit for a year or so to "digest".
Depending on where you are, and what the orientation is, this looks like a great start for a "hot" cold frame. Historically in some parts of the world, they would dig down 12-24 inches I believe. Fill it about half way with fresh manure. Fresh being important. Then cover it over with dirt. As the manure composted, it would heat up. They would cover this over with a traditional cold frame of some sort. If timed right, they could start things in that cold frame due to the
root warmth and cold frame far earlier than they would otherwise be able to.
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