I added the configuration we used to create levels for the game to the download list, since I've already seen an attempt or two at custom levels, and figured I might as well! The setup is a bit janky, but it gets the job done.

Basic tutorial:

1. You will need to download Ultimate Doom Builder.
2. Extract the contents of the Mapping Config zip into the Configurations folder.
3. Open UDB, go to Tools -> Game Configurations.
4. Click GZDoom: Snap (UDMF), and check the box next to it.
5. On the Resources tab, add snapgame.ipk3 as a PK3 file.