Just an idea -- I have a very tall tank (30"), and I knew I'd have a hard time stacking rockwork without ending up with a giant pile or wall. I decided to buy some fiberglass rods (driveway markers) at Home Depot and drill a bunch of my rocks so I could stack them in columns. I used a hacksaw to trim the rods to size. Any rocks with natural holes could be stacked too; it was just a matter of playing with the rocks, seeing which ones went best with which, to fit together a few columns. I tried to put larger rocks on the bottom, and alternate so each rock stuck out in a different direction as the column progressed (this leaves lots of space for corals and fish). I ended up with three columns of different heights/widths, but in a cube you'd probably just want to have one central column, and then you can place rocks around/beside it.
None of my rocks are permanently epoxied together -- some have little bits of epoxy strategically placed where I've bridged rocks between the columns, just to ensure that if something is bumped things won't shift, but they would come easily apart if needed. The columns can be dismantled by lifting the rocks off of the central post; they can also be slightly rearranged any time -- all I need to do is lift the rock above, and I can rotate one part of the column, then let the top back down. All the rocks are free to spin around the post, but gravity keeps them in place. Everything is very secure, until we want to move things around and then it's actually quite easy to change.