We used "CLECO" style fasteners to hold the part/parts in place - then tack-weld, etc.
Tim
How did you use clecos to hold the stuff in place if you are using a butt joint?. Most clecos are thru-hole for lap joints, although I've seen some that are clamp style which wouldn't work except at a panel edge.
One could use the
Eastwood panel clamps (also available from HF), but with a non-accessible back side, those wouldn't work, or if you could get them in place before closing up, you'd lose the little wedge piece which would wind up rattling around perpetually at the bottom of the space.
To the OP: You'll just need to cut a few exploratory holes to see what's behind the corners. Usually not an issue as rust has likely already opened a few holes.
