I agree that it would be better if the Dec axis could be stored sideways. There are other brands of these heavy duty toolboxes. I think Milwaukee has a similar set, but I am not sure what the dimensions are.
The footprint of the Dec axis with the DOVELM162 attached is 12" X 16" X 8-1/2". Allowing for at least 1inch of padding all around it looks like a Pelican 1607 case (inside dimensions 21.1" X 15.8" X 11.6") would accommodate the Dec axis plus saddle in a sideways orientation. Pelican sells their cases with a no foam option. You would probably want to use the high density polyethylene foam and cut it yourself.
There was another discussion about storage cases on the forum not long ago, and Roland commented that you need to make sure that the gearbox housings are not subjected to any stress when the mount is being stored.