I did eventually get parts for both sides. One of the flexible arm clamp tripod mount thingies was not stiff enough to hold the keyboard up, but the other one was great. Since they were both the same part from the same order... it's kind of a crapshoot as to whether or not it will work. But I will continue to experiment.
Honestly, I measured the hole spacing as 19.5mm but it is definitely not quite right because it is tough to get all 4 screws in. I don't have gauge pins to measure it correctly, and the accuracy suffers. But you can get two screws in which was enough for my use.
I uploaded the Fusion 360 file so you can easily correct this.
I would also print it upside down from how it displays on Thingiverse. That way the mount <-> keyboard interface is as smooth as possible. (Well, if you have a smooth print plate, I guess.)
And how did you bolt those things on? Are there screw holes on the back?
Here is a Thingiverse link: https://www.thingiverse.com/thing:4769961
There are caveats mentioned on the Thing page. I measured the screw spacing imprecisely and you will have to pick your favorite diagonal to put 2 of the 4 screws in ;) You'll also have to mirror the file to print one for the other hand.
One thing I thought I hated about the Ergodox was how inaccessible the small keys on the thumb cluster were, and I thought I would like 3 big keys better. It turns out not to be the case... those keys are useless, but there are a lot of useless keys that are nice to have around (arrows, delete/insert, home/end, pgup/pgdn). I mostly do everything in Emacs and so don't use those keys, but when I'm using non-Emacs software, I do miss them.
(I ultimately decided on Backspace, Alt, Up; Space, Enter, Down as what I use the big thumb keys for. Not sure I love this. This transitioned my backspace key from my right pointer finger to my left thumb, and it broke my muscle memory. I still forget that you can control backspace words to kill them outside of Emacs (where I use C-w for that, which closes your window in every other program).)
TL;DR: it's fine! But still not perfect.