A trick I figured out from desperation once....
Buy Sq-D ground bar kits, they are available in 3-hole thru 23-hole. They also include a unique sort of triangular cross sectional tapered self thread cutting screw(s).
Buy 3/4" x #8 self drilling screws (self tappers), these will be useful later as I will explain.
Now you have a ground bar and self drilling screws but one is not allowed to be used with the other right?.
However in this case it is exactly what you need, espicially if you have a box/panelboard/gutter/whatever mounted to concrete. There is no way to drill and then tap a hole in boxes mounted to concrete?....there is.
If you position the groundbar where you want it and then use the #8x3/4" self tapper to drill the pilot hole the Sq-D provided screw(s) will cut thier own threads and suck up as tight as it is possible, you'll strip out the screw head before the the screw strips in the can hole you have made.
Anyway the basic trick is to use a #8x3/4" self driller as a pilot hole for the Sq-d groundbar screws,it works and it works great. You dont need a tap, you don't need to pre-drill a specific size hole knowing your bit will run against concrete and be ruined once you punch through.....
If you hold the groundbar in place and use it as a stop then only the cutting head of a 3/4" self-driller will go through, no threads will catch.
I have used this over and over again to ground existing pull boxes, gutters, and older panel cans, it WORKS!!
Hope this helps.... Greg