How to cut 2-1/4" diameter hole in sheet metal box?
I cut holes in sheet metal with hole saw bits all the time.  Depending on how thick your material is you might need more than one hole saw to get through it in a reasonable amount of time.  22 gauge is certainly no problem.  Not sure how thick your sheet metal box is.  You want the ones with the most teeth but generally these are the cheaper ones so you should be fine.  I am not sure I'd try on 1/4" plate steel but sheet metal shouldn't be a problem.