Because of a 3 month backorder, I designed a water-cooling jacket for my RC boat's motor. This design is for a 36 mm diameter RC motor and is 40 mm long. It requires 2 o-rings and two barms and I have linked the ones I used below.
This design worked so well that I have used it in two boats!
RC Motor: https://www.amazon.com/gp/product/B07WHCM3Y2/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1
RC Boat Design: https://www.thingiverse.com/thing:3328520