Spacing a brake rotor out 15mm?

I want to retrofit my fat tire, 175mm dropouts ebike from a Bafang rear geared hub motor to a direct drive higher power motor for a bunch of reasons. I bought a 175mm dropout spec motor that fits the bill perfectly on the 7-speed cassette side but because it is thinner overall, the 6-bolt brake rotor hub casting is about 15mm too short. I've found up to 7mm spacer blocks but am wondering if using such a large thickness spacer (15mm) is something that is done successfully and safely - more or less industry standard practice? It seems like there would be a lot more torque applied to the extension and 6 bolts than is normal. I have a good size lathe and could make my own spacer if needed but I don't want to be doing something that is not structurally safe to do.

I'll admit up front that I don't know the answer. But I'd observe that the stopping power of the rear brake isn't particularly important and does't experience the same level of stress as the front brake. If it were me, I'd make a beefy spacer out of a strong (6061 T6?) aluminum, use good quality bolts that seated well into the hub and feel pretty comfortable.

Alternatively, have you checked to see if there are adapters that will allow you to move the caliper assembly inboard that amount?

I actually found a commercially available, specialty 15mm Alum spacer with the proper bore and Stainless bolts.