A lock/wedge to keep the slider in place, so it doesn't pop out of its socket.

If you have this problem, the sliding arm has probably retracted and is hiding. Possibly you can't close the box at all, since the un-slotted arm is jamming the mechanism. 

Shove your fingers in there, find the wheel at the end of the arm (see pictures), and slowly pull it out to its socket. If you don't lock it down with something like this model, it'll dislocate again fairly soon. Taper etc won't work, I've tried.

Ford Fiesta MkVII (2017) Titanium. Probably fits other models as well.

According to the dealer, this part can't be replaced with less than the whole glove box section.

Instructions:

Print it sturdy. Supports needed due to overhang.

Reattach the arm/wheel in its socket beside the glovebox.

Jam the new part in there.