I currently store my modular in the Billy book cases. Interesting work-around but too permanent for my taste. As my modular grow, I was swapping around and given some are much taller than others, that required me to relocate the shelves to different slots. Wouldn't look good having a random slot half way up a shelf.
For mine, I simply built the case with a couple of washers sliiiiightly separating the vertical sides from the top and bottom. Sure, there's a gap a bit bigger between where the doors meet but only a little bit. I simply ran a line of celotape down it then used a sharp knife to slice it so it acts as a dust barrier (as I do on all my cabinets that display lego to reduce dust ingress).
It honestly works much nicer than cutting lines in my opinion.
I can't remember how to post images but if anyone wants to see, I can find out and get them up.