7mm Hex +6mm (extension)

Should fit most micros. I used this on my FMS fcx24 K5 to add some space between the rc4wd rims I want to use, and the portal axles.

There are two versions. One is dimensioned exactly the the stock adapter, the other has added tolerance for easier fitment. Print one of each (~3 minutes) and see which fits better for you.

I used PLA+ from polymaker, but I also printed on in LW PLA for funsies and it worked fine, so use whatever filament you want. Let me know if you have questions or need a different size.