re: Storage out of flat bottom bench seat?

It's not the end of the world to cut it, but it's a really good idea to frame it with something to prevent stress on welds elsewhere. Like I said a few lengths of angle and some pop rivets will make it come out professional looking. This is the closest I could find to an example of how I would do it, frame the hole with the angle, cut it to 45s at each corner, and then pop rivet the angle all around. It will be just as sturdy as the original sheet, then you can add a plywood lid or really any kind of hatch you want.