Drill cordless or otherwise with a drill bit that is exactly the recommended size for the rivet nut you are using.
How to make your own towers for roof rack.
If you have factory roof racks that run from front to back you can build your own steel crossmembers using one inch square tubing cut to size and secured to the factory racks using carriage bolts.
The tolerances for the nuts are tight and if you use a bit that is slightly over sized the nut won t crimp properly and you ll be screwed.
They re extremely easy to make and pretty cheap.
Remember you re drilling holes in the roof of your vehicle.