But you can use a right side shift key from an standard M. You'll have to cut it down on the left somewhat but the position of the stem is correct for everything to the right to line up. I suppose you can repair the left side of the key with putty or epoxy, maybe get the original left side back on...
This is a better approach that putty filling a stepped key because it maintains the proper surface on the top of the key.
You can try to get a blank one from Unicomp. It's not listed on their site but I have to believe that if you contact support they'll be able to set that up.
If they don't, you can buy their Geekhack shift set (http://pckeyboard.com/page/SBLE/GHShiftKeys), or the RGB unprinted set they offer (http://pckeyboard.com/page/Buttons/RGB).
Edit: Realized that you can also just get a standard printed one from Unicomp or elsewhere, because cutting off the left side cuts off the text on the key.
Thanks for the tips!
Ordered a bunch of RGB key caps this past week (the mods aren't compatible with my Ultra Classic 104, but I just needed the right-shift keys), & they arrived today. Naturally I couldn't wait to get into it. Photos/video:
https://www.flickr.com/photos/absfackeycap/sets/72157643488205464/Mostly it works great. There are just a couple of minor caveats.
1. Obviously there's the cosmetic issue. I'm going to invest some more time later sanding down the outer edge and seeing if I can trim and epoxy-glue the left side of the cap onto the sawed-off end.
2. A little more annoyingly, if you compare the videos shown in the last two entries in the album, the modded shift definitely wiggles from side to side more than my original stepped caps lock. It took me a little while to pinpoint the exact reason for this. At first I thought it was the angle at which my finger hits the key. However, on closer inspection, I have realized that the stem width of my caps lock is very slightly but measurably wider than the stem on this green right shift! I was pretty surprised by this and I haven't determined yet if this is normal manufacturing variation or an intentional design difference.
However, neither of these are really dealbreakers. The cosmetic issue doesn't bother me too much (BS boards aren't really known for their cosmetic beauty anyway (/puts on flame retardant suit)). The wiggling is annoying if you focus on it, but in ordinary typing it's completely unnoticeable for me, especially since when you're banging on a BS board you're using a lot of downward force anyway.
More to follow after I've have more time to mess around. Still, my initial report is that if you want to dive in with buckling springs but can't stand stepped caps lock, this is an easy and pretty cheap ($9 for the RGB mods set) solution.