I’ve never read definitive info that says you can’t. Are you concerned with the look of having the spool on the outside or trying to figure out how to get guide it into the box? This is my solution using JonathanB’s stupendous design. https://grabcad.com/library/hotbox-diy-enclosure-for-3d-printing-1 I actually much prefer having the filament drop straight down into the extruder and i’ve tried many different options. And here’s the great thread in this forum. HOT BOX 3DP ENCLOSURE SYSTEM (For Taz) It’s a must read.
I’ve printed for 7 months with only an intake for the control box and even though i built the intake and the exhaust ports from Jonathan’s design, i’ve never actually wired up the fans and used them. In other words, it’s definitely never been too hot in the box. I print majority in ABS but when i print with PLA i just leave the door cracked so it doesn’t get so hot.
A filter would be just fine to use as long as it didn’t restrict the airflow so much that it caused stress on the fan. You’d have to be the judge of that.
It’s hard to explain but i actually bring my cables in from the floor of my enclosure. The base of mine is a bit bigger than the file cabinet i have it sitting on and i’m just exploiting the fact that heat rises and won’t escape from the opening in the bottom. Never caused me a problem. But there are also a lot of these types of brushed openings you can get as well to keep the heat in and make an easy way to pass cables through. brushed grommet - Google Search
Good luck.