Most of you I guess use wood screws and wooden rails to mount your modules, and if so, you can safely read no further.
For those who use extruded rails with nuts (or threaded inserts) and machine screws:
Mostly I’ve used either pan head or socket head cap screws. But Befaco sells “knurlies”, these lovely things:
I have a few (like about 6) of these and they’re wonderful. They’re also 38,70 € per hundred, or $40 at SynthCube. (The link is to the M3 version but they also come in M2.5, which is what I’m chiefly interested in.)
I figured these must be something that’s available in the non-Eurorack market for less, but haven’t found quite the same thing. Have you?
About the closest I’ve found are these https://www.aliexpress.com/item/4001289196903.html.
Head diameter looks to be about 6 mm, compared to about 4 or 5 mm for Befaco knurlies. Lot of 190 (seriously? 190?) for $34.25. I don’t see these in smaller lots. I do see many vendors with these: https://www.aliexpress.com/item/32888243969.html.
5 for $1.70, so about the same price each. There the head size is 9 mm for M2.5. I have some of these, both M2.5 and M3, and they’re pretty good.
Problem is: the Kosmo Specification says:
That means any head diameter above 6 mm won’t work on adjacent modules (unless you leave a gap). Knurlies will fit although they’re very close together. By the way, I note M3 square nuts are 6 mm wide, so just barely work for Kosmo modules.
In Eurorack it’s common to have slotted mounting holes which allow for some flexibility in mounting. Typically I think they’re 1HP (0.2" ~ 5 mm) wide and centered 1.5HP (0.3" ~ 7.5 mm) from the edge. So there’s a good deal more room for wide heads (and nuts) there.
I’m thinking of going with slotted mounting holes for my future FR4 panels, with one end coinciding with the Kosmo spec position but extending inward a few mm from there. (Pretty sure JLCPCB won’t do plated through slots, but I would think grounded pads on each side would be possible.) For homemade panels I may just switch to drilling holes further from each edge. For Kosmo panels from LMNC or others, well, I may just be stuck with not being able to use wide heads everywhere I’d like (or with drilling new holes).
Any further suggestions or ideas?