They probably will not grip the rectangular hole right since they are designed for use in a round hole, You may have to drill new holes.

 

FWIW see the following post for a plastic insert that sorta works ( Auveco Nylon nut ) s.

 

View attachment 212524
This rivet nut is the best option I found so far

Maybe wiggle one of these in there and use glue to hold them to the back of the sheet metal?

I have rivet nuts in these slotted holes. I have no expectation they will handle a large load but they have held up 8mm ply trim peices for several years.
8mm instead of 1/4" because it was free.

 

Unless the van is in a crash the, the load is in sheer. If the rivnut is at the bottom of the slot then it can't slide down due to shear plus in that location it has 3 sides to grab on to. If additional tensile strength is desired a second one could be added at the top of the slot.

 

...
This rivet nut is the best option I found so far

Maybe wiggle one of these in there and use glue to hold them to the back of the sheet metal?

Why would you not drill holes so that they are actually strong? Slotting into these will yield a weaker mount point 100% of the time versus a correct-sized hole drilled.

 

Except for the boron steel parts, any reasonbly strong person can rip a rivnut out with a little leverage. Sheetmetal in modern vehicles is foil thin.
I used the slots because they were exactly where I wanted a fastener and the loads the fastener would be subject to, the slotted hole was good enough.
Good enough will often serve as well and for as long as the "right" way with much less effort.
⁷

 

I think we are back to rivnuts versus plusnuts argument. Plusnuts have the expanding fingers that might grip in a rectangular hole, Rivnuts do not/will not.
The question was asked about rivnuts.