Cut some sheet metal and patch it up. Ues a silicone bronze rod tig weld for flexing so it wont crack. Then get some auto seam sealer and finish it up. Don’t just mig it. I see a lot that just mig it but it will just crack all the time and be very careful of the expandable foam. Most have a very low flash point and put off a gnarly smoke.
Everybody thinks the expanding foam is a good idea till they have to work on a truck thats had it done. Would not recomend.......
dap makes a water based expanding foam. You can get it at home depot. Check the label first.
Poly foams require moisture as a catalyst. It doesn't work well in tubing, it can actually remain liquid in the middle until the day you need to make some changes to your cage. DON'T DO IT! The acoustical benefits are too little to justify all the associated problems. If you just can't help yourself, then at least buy the 2 part type of foam. It comes in a box and I don't think they sell it at home depot. I'm sure mcmaster-carr carries it.