center of a 1x1 room doesn't do it.... do this test for me, walk near your box, then walk away, and see when the "search" text disappears, that's how far away it needs to be, a 1x1 house won't cut it
It can even be a bit further than this, since the server appears to give a little leeway (easy to test by tying up most of your bandwidth to make your connection laggy, then open a box while moving away from it) - it's somewhere between half a foundation and 1 foundation's width. If you have a box/furnace/etc on the innermost (4) corners of a 2x2, where they intersect, then you should be safe from cheaters attempting to steal through your outer walls. If it's a Large Wooden Box or Furnace, though, they may be able to loot it from the roof if it's only a 2x2x1 (single-level) building. If you've built 2x2x2 with storage only in the middle bottom floor, they should have to breach with C4 to get to it (assuming your doors are metal). Of course the more walls and doors between your stash and the outside, the better! Hope that helps ;)