i love the design of this telescope however, the primary mirror is in the open and can be easily bumped or scratched. this sleeve fits nicely in between the rails and allows the mirror to be protected from outside ambient light sources and also from being damaged. i made it a bit bigger than the build area on the ender 3 pro so just scale the z axis accordingly in cura to the size you like.

i did 10% infill
100mms
.04 nozzle

very sturdy yet light .