Just like a waterbed, you'd better make sure your floor will support the weight. Conventional safes sit against a wall, where the floor is better supported.
We found two 3-drawer units at a used furniture store that fit nicely under our queen-size mattress, with a 16" gap down the middle. They look like motel take-outs. The drawers are great for linens, and the space between holds the overflow from my locking gun cabinets.
the bed is more comfortable, too. Box springs are a waste of space.