Use a UV/weather resistant filament. I suggest ASA-CF, PC-CF or ABS-CF. PETG-CF is UV resistant to some degree (better than PLA). 

Designed to be used with the LWS category of camera rails/rods. the rods are 60mm apart and have a 15mm diameter.

The part is designed to snap in to place on the rail without needing to unscrew anything or disassemble your rig. This makes it possible to swap out these lens holders to fit different sized caps on-the-fly