Mounts; Motors, bearings etc..

[JAZZCNC]

Don't think you'll need as much room inside the box section as you think because you only really need say approx 10-12mm more than the length of the coupling because the shafts of both motor and screw go inside the coupling. So the only extra distance required is the thickness of the coupling dampner (If using split type coupling) plus a bit of wiggle room.

Obviously will depend on how you mount the fixed end of the screw IE:bought BK/BF blocks or DIY etc and how much of the screw you let stick thru the steel.

Edit: Drawing or rough sketch of intended frame and screw mounting would help. ;)