+1 for Paul's suggestion.
Another option if space allows- drill 4 corner holes & a large central hole, step drills work perfectly for this. Using Lt & Rt offset snips make a series of cuts radiating from the center hole towards the corners till you have the square hole required. Quick and slightly less messy than the dremel cut.
built a few RVs, rebuilt a few more, hot rodded more, & maintained/updated a big bunch more