Question is in the Title. I did read somewhere that the housing for the Makita is 65mm I.D. and that the Open Builds Spindle Mount is 71mm I.D. True? I do have the Bosch Colt but the housing on that is utter garbage.
The Makita is 65mm. You can get 65mm spindle mounts from all kinds of vendors -- mostly from Asia. But, if you want one quick Amazon may be your best bet. Or, 3D print a shim for the OpenBuilds model. Spindle Mount Adapter for Makita RT0701C