An American penny will fit but use a pre-1982 copper coin, which smugly fits the F and A series Canons, Nikon FG and EM, Minolta X-370 Pentax K100, Yeshica, slr, Olympus 35 and XA/XA2 and the Zenit 12 XP, though it does have a small ridge which can easily file/sanded down for a smother fit.
A set of Dime, Penny, Nickel, Quarter of American coins with mounting holes drilled for on a key chain, should fit most needs and a modified screwdriver or small stiff, mild steel bar with a coin fixed either at the end of a vertical or at the end side of either leverage tool for stuck caps. Alternatively, a .5 inch or 12.5mm flat head screwdriver or slightly larger, at the tip, will grind down nicely, however I suggest cutting off the thinner tip, and moving the beginning grind up the blade until it is in metal the thickness of a penny, giving it it's arc and hallow grinding both sides just shy of a penny thickness.
If the majority of your cameras will take a penny grind, this is the simplest way is to modify a screwdriver, as others have suggested, but I still recommend using heavy steel shafts in square profile, as, when needed, you can clamp a Vice-Grip to that shaft or just use pliers, to give extra force to a frozen battery/motor cap.
If your screwdriver has bolstered handle cap, like this set at Harbor Freight, you can use a socket and rachet to apply torque,though I think the round shaft and blade on the largest is too small so heavy cutting and grinding will be needed.
Personally, I do no care for forcing a cap off with that much pressure but better to replace the cap and rechase camera threads then let Heaven knows what be destroyed by batteries and their corrosive content.
Cheers and Godspeed to all.
IMO.
IMO.