Print in black fillament, suggest a 0.4mm layer height. To print in one piece you need quite a tall printer (like the sidewinder X2) but it could possibly be split and printed in two parts then glued together.

This fits inside the tube of the telescope, can be attached with some cable-ties (or drill some hole sin the spider holder and use screws). Slides in when the scope is collapsed and blocks stray light when it's in use.