Drilling steel isn't too difficult if you have sharp bits. If I were you I'd buy a good 1/8" bit for drilling pilot holes, and some bits in whatever size you need. You can always sharpen them again.
I've drilled those exact cases with a Unibit without too much trouble. You might need a higher-powered drill depending on how much torque your cordless can deliver... I've seen some cordless ones that are pretty juicy so maybe you'll be fine.
definitely try to start out with a pilot hole and keep the bits well lubed. i will have a cap off a quart container of oil half filled and dip the bit in there prior to drilling the hole. when using the larger bits i will do a few drops of 3 in1 oil while drilling. also if you dont have one a drill press is the way to go. i got my little delta brand new for under $80 about 15 years ago. i am sure one can be had in the used market for a reasonable price. also new or well sharpened high speed steel bit are best for metal cases but i have had good luck with the cheap reconditioned carbide bits for drilling my pcbs.
I had a steel scratch plate drilled out for my guitar, I just took it to a local hardware type shop and they drilled out 3 holes for about £2 so that could be a possibility if your near anywhere that might have a decent drill, just an idea.