Is the rotary dial mounted entirely to the 3D-printed part, or does it sandwich to the aluminum panel at all?
3D printers are great, but caution must be taken when mounting something that's going to be loaded during use (even by a hand resting on it). The most common material for 3D printing is PLA, which has a softening temperature around 55-60C. That is easily achievable under a bubble canopy parked in the hot sun in summertime. The heatsink on the back of the dimmer suggests there's a fair bit of heat there as well, which may also contribute to softening.
There are materials better suited for this (ABS, PETG) but they are a little harder to print. Achievable with a home printer, it just takes work.