I made an adapter for the DeWalt DWP611 router to fit an 80mm spindle mount. This is designed to be flush-mounted in the Sienci Altmill MK2 80mm spindle mount. 

This part features an offset mounting configuration that allows more space for the DWP611 motor housing. The gap is designed to face the “front” of the machine and also includes a notch that aligns with the Altmills mount (see pictures) to help with alignment.

It may work for other brands, but I didn't put in any design consideration for any other machine mounts.

 I haven't done any extensive testing, but this should be printed 100% infill with a rigid material. I used PLA for my part, and the fit was good. Due to the thin design, I advise using a brim to help with bed adhesion.