Coincidentally, I've been working on designing cherry-compatible 3d-printed keycaps just lately. (Just last night, in fact.)
The switches you're working with appear to be different than the ones I'm dealing with. (Are your switches like this?) But for future generations, the measurements I've found that work very well with my Glorious Panda switches:
+----------------------------+ -
| - __ | \
| / | | | |
| | | | | |
| | ---- ---- - | |
| 4.5- | | >1.2 | > 5
| | ---- ---- - | |
| \ | | | |
| - |__| | |
| | /
+----------------------------+ -
| |
\____________ _____________/
V
6.8
(All measurements in millimeters.)
For Glorious Panda switches, you'd want to leave at least 3.8mm for the stem to extend up into the baseplate, but your switches look low-profile, so it's probably a little different. I'm guessing some of the other measurements would still apply, though.
I printed at 0.12mm layer hight with Hatchbox PLA on a (slightly upgraded with some metal replacement parts and a CR-Touch) Ender 3 Pro. And it fits nice and tight (though I can't (yet) vouch for how long it would stay tight; it might be that the PLA would deform over time and get loser. Time will tell.)
I'm honestly pretty new to the mechanical keyboard game, so I wouldn't know much about where you might purchase something like that. I suppose one option would be to buy a keycap and cut it apart to get the stem you want, but you probably wouldn't want to buy a whole set of keycaps just for that.