24" is the standard cabinet width.
Also, MDF isn't expensive, just buy a second sheet.
Also, with a lot of wives I think the real problem isn't the number of cabinets, it is the god awful mess, racket and time you spend making them. That part all still happens if you build one for a friend, your wife might even approve it, you will still get all the negative repercussions anyway. As a guy with a lot of games who has friends with a lot of games, it isn't the basement full of games that drives them crazy, it is the game parts in the upstairs closets, the dead cabinets under the carport, the monitor chassis sitting on top of the washing machine and any other game related mess that drives them nuts.
Also, woman vision sees games as taking up massive amounts of space because they are tall.
Btw, the best way to sneak in that second cabinet is to first get some worthless piece of second hand furniture that is exactly the same size as your next game, put that in next to the other cabinet and let it sit there for a few months not getting commented on at all. Then replace it with a second cabinet ("what, it is in the same space as that little table you never really liked").