Developer Cyril Venditti recently decided to open source many Neo Geo and related projects. Some that might be of interest to people are a custom battery holder, a SNES to Neo Geo controller adapter and a Supergun. All are released under the GNU General Public License v3.0, so people can feel free to make them at their own risk. Check out the link to see everything available:
Cyril’s Projects: https://github.com/NEO-JAMMA/Neo-Geo_MVS_Projects
Cyril also uploaded a design that makes creating a GBS-Control a bit easier as well, which may be worth taking a look at. I believe other people have also open sourced similar designs, but I’m always appreciative when people take the time to open source theirs! I think if nothing else, the battery holder is a great “first project” for people who want to get into making circuits by themselves and is pretty useful!