There's usually a very good reason, from a ferret's perspective, why one corner is better than another. It's a good idea to have some newspaper or something just next to the boxes, as they'll usually wipe their butts there after using the box. If they persist in using a place where you can't put a box, try to put something that smells like it's definitely a toilet, like a favourite sweater, there for some time. I use a box filling used of granulated recycled newspapers. If you use sand, don't use clumping sand, as it's likely not good for the ferrets' airways.
Wood chips is totally wrong for ferrets, either as living material or litter box filling.
We've got four boxes where we live, which isn't a large place, and I usually empty two of them twice a day. There's thus always one suitable for everone. When I empty them, I don't empty them totally, I only remove what needs to be removed, usually that closest to the wall, and I move the rest of the filling in that direction. Material only needs to be added once or twice per week. In front of some of the boxes are heavy things, so they can't move the boxes and use the area between them and the walls, which apparently is a completely reasonable thing to do.