This 2729’s first try at a custom encoder mount for 8mm shaft motors (it works with both the mini CIM and CIM). We CNC milled it out of 2.5" diameter delrin stock ‘pucks’, cut about 5/8" thick. The milled channel perfectly holds a CUI AMT10 incremental quadrature shaft encoder, while still provided a solid mounting surface for the motor.
The AMT10 has a configurable resolution ranging from 48 up to 2048 at a max of 7500 RPMs which would work great for either motor, and is about $24 from Digikey.
We discovered the AndyMark CIMcoder after designing this around the AMT10, but honestly the AMT10 provides configurable (and higher) resolution at less cost than the CIMcoder. [cost of CNC mill not included ]
Best of luck to everyone this year!