Stimpson GS-3 eyelets are typically used.
I would get samples and see if the battery holder is actually practical. Many of them out there are stupid like this Keystone part.
1/2" long pins? mysterious center hole? If Keystone was smart they would put a hook for a PCB slot and then no fasteners needed. Or add a third pin at the other end.
The mounting holes are to the side of each cell so the fasteners could have thickness, but the center rib is there

The MPD devices are better thought out, the center rib is out of the way of any fasteners (screw heads). They suggest 2-56 or GS-3.
There is a lot of manufacturing labour to hand-install screws, lockwashers etc.
Consider using two single battery holders, they solder in.