Coming back to the "Coffin" - it uses less materials so you can use thicker/stronger ply/planking/board/metal. The lid can be made sufficiently thick/strong that you/others can jump up and down on it with no damage or access. As noted previously, it has a lower profile/draws less interest and gets over the planning permission issue.
To prevent it being rolled over/carried - you can stake it down internally/bury it/build up earth around the sides (and if you then throw a dirty old tarp over it it will look like a muck heap). give it buried spreaders (per van vault below).
hinge & Lock design needs to be thought about, to prevent it being smashed off, or fit an internal lock. (Have a look at Van-Vault for design ideas) If you buy theirs, get rid of the yellow paint and end handles, slide something long and strong thru the base "sleeves" (metal, concrete lintels, preserved wood) and part bury it to hide them.
Depending upon how much you want to store you could have several of these, with tarps on they would look like a series of muck heaps.