Power Cable Strain Relief Solution Needed for Restoring Vintage Electronics
3D printing in rubber is a thing these days.

Making a mould in silicon and then casting a strain relief in ureathane is a option.

Sugru is a hand crafted solution.

I used to fit strain reliefs manually on special orders that my supplier had custom made by the cable company. The wire was about 3.5mm diameter. They used a Teflon stick on to witch to over-mould the strain reliefs that they normally over-moulded onto the wire harness. We would slide the loose relief over the wire with some water or alcohol and then put a drop of low viscosity super glue in the joint to lock it in place. Your solution here is to look for a similar strain relief on some donor device and remove it from the donor cable.