I'm not sure if this thread is even active anymore, but I posted something in another thread about this. I would be afraid of the cube sections to build a cage because a) if it's the 1"x1" squares the ferrets would get out, wouldn't they? and b) if it's the mesh style kind, then wouldn't that be nearly as bad as using the aquarium in terms of ventilation? ... just a few things I would worry about. (not to mention one of the little buggers figuring out how to detatch the corners!)
Here's what I did:
I use a 54 gallon rubbermaid tote (approx 3 1/2' wide x 2' deep x 4x tall) as a base and attach hardware cloth with a small grid to the tote with wire. (I use zip ties to attach the hardware cloth panels to one another but the wire seemed more sturdy with the tote. I use the hardware cloth for the walls, ceiling, and "floors". for the floors, I folded the hardware cloth double for more support and also slid dowels underneath them to brace them. I cut holes in the mesh for ramps and also used the cloth for the ramps, but I taped all the sharp edges and coated the ramps in non-toxic glue to protect their little feet from rough edges. (wouldn't need to be done if you use coated hardware cloth, but it was much more expensive and not needed for the entire cage) I also found an interesting setup where the litter box is contained in a separate tupperware container and connected via tube to the main 'house' so the potty room is easy to detatch and clean more often. (http://wrongcrowd.com/gallery/litterbox
i hope this helps. I've considered other ways to build the cages, but this seems to be pretty sturdy and was only about $35. and-- if I need more space for any reason, just build another one and it can go next to it or kitty corner or however the arrangement and attach using the tubing or PVC piping.