Wow, Guyon, thanks for the great pic and the very kind and generous offer, but I found 5/32" to be just a tiny little bit less than 4mm. A 4mm hex key will also fit the bolts. But of course it would be pretty hard to find some nylocking nuts here... I might just use good ol' LocTite.
Ethan, thank you so much for using this particular size of bolts... you're a genious, man.
take care y'all!
maethor