That is darn little leeway, I can only think of one type of box that is listed for being installed in the ground & it is large & expensive.

How about Type UF cable? UF could be run up a post. There is a non-metallic post on the market that will conceal the cable where it comes up and the fixture can attach right to the post & it is available in a fairly low profile.

Tom

Few things are harder to put up with than the annoyance of a good example.