How big is the gap? Will crown, 1/4 round, 1/2 round etc do it? My only other DIY idea would be spray foam, cut it back after it is dry then mud rather then backer rod.

1/2" is the largest gap, get's smaller on different sides. Spray is an interesting idea.

I have used the spray foam before-window and door-low density because it cuts very nice and leaves a very smooth edge that in my instance I painted and called it done.

Is there a specific product you used?

No, just no name local hardware store brand but made for window and doors. It is considered low expansion foam.

I use the L bead with a tear-off edge. It will span that gap.

Hi< Maybe crown molding big enough to cover the biggest gap.

good Luck Woodbutcher

I would just use a simple 1x2 pine board or similar.

FWIW, you can get L-bead that will cover up to a 2" gap (Trim-Tex shower bead). But, in this case, I agree with Millworkman and would use pine to cover the gap AND the exposed metal tie.

Bartie Musa - This is very helpful thank you, dealing with a similar gap in my home.