I opened mine up because I preferred the 3.5mm plugs to the wires. The shrink tube is only punched where you see wires entering. The open sockets you see are shrink covered. Motor and negative battery hook to far side. Positive battery hooks to near side.
Here is what I found:
Anything that fits will work. I used some clear 1.5" tubing. 1.25" would probably be better but I couldn't find any.
That said, the MGM controllers don't seem that well environmentally protected. The shrink tube is applied before the heatsinks are mounted. The area over the FET's is cut away before the sinks are mounted. Leaks will occur around the seam. Both ends are largely open.
It also appears that the coating is missing in the inboard sections of my "marine" controller. I.E. the center of the boards are bare. I had a bottle of mil-spec acrylic coating so I gave the boards a good dunking before re-applying the shrink wrap.
Before applying the wrap I cut a plastic end cap for the data port end of the controller. I punched screw holes (only) through the shrink wrap instead of removing the area over the Mosfets. I am sure the shrink wrap adds a little thermal resistance but the FET's already dump heat through lead frames, PWB's and epoxy encapsulation. I doubt the added barrier makes much difference. I would rather have the environmental protection.
I used a little coating on the seam between the shrink tube and the end cap. I still have to RTV seal around the caps. A little tape over the program port and it will be pretty well dunk proof.
'nuther view: