Basic security.

[Luci_ferson]

We dont have this problem as we live on an ancient loch beach, so its very sandy about three feet down.

It sound like you are going to be doing a bit of drainage, dig down a bit further than you need to and build it up with gravel, putting a drainage pipe in or below the gravel should help,...difficult without seeing your ground

Luckily for me my ground isnt too bad as im near the top of a hill.
as long as i only go about 2.5 to 3ft down water shouldnt be a problem, but it means il have to build my box above atleast 2 to 2.5ft so that theres a decent headroom once in.
but ive got that sorted. the hardest part for me was the idea, and you sorted that out.