Gloomhaven is notorious for having to keep track of lots of stuff. This holder allows you to display the monster stat card and attack deck for a session of Gloomhaven, and free up a bit of horizontal table space. Most sleeves should fit. This remix of an earlier design has a slightly large base for better stability.
The design was originally published to Thingiverse under a Creative Commons Attribution license as Item 3778646 by Feday.