Cutting a patch panel to fit perfectly

This is my first time doing any kind of rust repair like this and I'm that guy that screws up and ends up cutting the patch panel way too small. What I'm doing is I'm replacing part of the trunk floor to the right of the wheel well and over to where the floor meets the trunk extension.

Are there any tricks to cutting the patch panel to perfectly fit in there to be butt welded? Do you guys just lay the patch panel on top and then go underneath and mark where to cut...and then cut a little bit wider than that and then slowly trim to fit? I figure either that or just slowly cut the patch panel, lay it on top and guess and check. I'm just not sure if there is a quicker, surefire way of getting the panel trimmed easier so I figured I'd ask before I got started on it.