Yeah, I ran into a similar issue before. What worked for me was adding a 4 mm thick backing plate behind the motor mount — it gave it some extra stiffness without adding much weight. Personally, I’d steer clear of rivet nuts, especially in setups with a lot of vibration. They can come loose over time unless they’re installed just right. If you can line up the holes accurately, bolting through a second plate is a pretty solid and easy fix.

Another thing you could try is adding a small L-bracket or gusset under the mount. I’ve used 1.5 mm aluminum for this — super easy to bend and you can just rivet or bolt it to the base. It’s super lightweight but does a great job at stopping flex. I’ve done this in some small automation setups where keeping things light and easy to put together was a big deal.