Tonight I fabricated the first of several stainless steel firewall passthroughs (this one for the starter cable). I made the inner hole a tight fit on the cable and long enough to stop flame passing through. The front and back of the inner hole is radiussed to avoid damage to the cable insulation. The outer diameter was large enough to machine a decent doubler flange for the aft side of the firewall and a locator groove for an E-circlip on the firewall side. I left space on the shaft for silicone and stainless steel washers to keep tension on the bushing and stop the firewall hole from elongating with vibration.
The others will be faster to produce now I know how much space to allow for the washers and E-circlip, but the larger versions (for bundled wires such as the 12 CHT/EGT thermocouple wires) will have space for 3M fire paste on the front side (which expands if the insulation burns away).