Here's my back shell for the AMP/TE Superseal 1.0 34 way connector which is commonly used by performance ECU manufactures.
There are two models here, one to accept a 222K163 Raychem boot, the other will allow the use of a regular piece of heat shrink. The hole in the one currently uploaded is 10mm, this can be increased very easily, let me know if you'd like to see other sizes.
My only concern with the design is how it will work with splices, as I build the loom for my car I'll better understand how that will work and adapt accordingly.
As for the one which accepts the Raychem boot, I know what you're thinking. Well that's not going to work, because as soon as you try and recover a boot over it, it will melt, and you're right. I tried, and just as the boot fully recovered the part melted. Perhaps if I'd been a bit more patient it might have worked better?
I printed mine with ABS, I don't have the ability to print Nylon yet, but I suspect it would work in that material. The regular piece of adhesive lined heat shrink recovered just fine without causing the part to deform.
If you would like a backshell which will have no trouble with the recovery of a Raychem boot, see the one I used as inspiration here, https://racespeconline.com/products/racegrade-amp-backshells
When printing, be sure to rotate the part to print bottom up.