I have modelled a motor mount for use with the Argos value drill motors and gearboxes.

I am using them for a non-robot project so the fixing flange is to flimsy for robot work but a few minutes with a STL editor will allow you to replace them with more durable options to suit your use.

I thought it may be of interest to people on here especially saince the drills in price have just had a 50% price cut.

https://www.thingiverse.com/thing:4111198