The MLOK spec T-nut is a very specific design with a designated screw size. That’s if you want MLOK capability (easy mounting). That said, you could use any basic T-nut if you have room to get it in the rail. There’s about a million different types, just google “rectangle nut” and find the size that works for you. I’ve installed several accessories on MLOK rails that way.
I found some that take M3 screws. I'm not sure if the cam portion will set right in the M-Lok hole. And they seem a little bit deeper than mapul's. But I may take a chance and order some. And/or I may try the rectangular nut.
m3 would be a good choice for a metric size that is smaller than the std screw (5/32?) size that magpul uses. Might be a 10/24 or something like that, sorta oddball. M4 is similar in size to the magpul supplied screws. Amazon has some m3 t nuts, just google that, m3 t nut, think it was the 1st result.