Thx, that's what you get when you let me have a little brainstorming.
Though the whole endeavour was kind of mute given the fact that I ordered a third HDD caddy yesterday and the guys are sending me another 10 slide screws along with it for free.
With those I already have this gives me a total of 17, which is perfectly enough for my current three 3.5" HDDs plus one 2.5" SSD, the latter of which I'll mount in one of the sleds using adaptor brackets.
I'll have to add a fourth 2TB drive soon though, so it's still good to have the homemade HDD screws.