I felt that the WMR motion controllers slid around a lot in my hands during fast movements, like in Beat Saber.
I designed a slide-on grip that could be 3D printed to add some contoured points to improve the grip.
It's a tight but not unreasonable slide-on fit and in several years I've never had one come off inadvertently. It also fixes a secondary problem where the battery door could become dislodged in the middle of things as well.
I printed in blue Hatchbox TPU for the bit of flex needed to get them on and the extra little bit of stickiness that type of plastic adds.
It's a symmetric design, so no need to mirror or flip or anything. Just print two and put them on in opposite directions.
STL file available via Thingiverse.