I agree, JB weld could be the best thing you could use. Having said that, it could be pretty messy to use that if the hole is hard to access. And I think you would want to avoid having any access go inside the hole, especially if it goes up against the copper/aluminum heat exchanger. With vibration, that could damage that coil.
I’d probably make a patch and glue that down. You can use a piece of tinfoil, a piece of sheet metal, or even a plastic or rubber piece of whatever. Tinfoil is super easy, but it’s thin. You can cut a little half dollar size piece of sheet metal, and bang it around with a hammer to the point where it folds itself around the hole. Don’t use a slick plastic like urethane, use a similar course plastic like the boxes made from. It doesn’t have to have any structural integrity, you’re just patching the hole on a low pressure air box.
Use some brake cleaner or something like that to clean all the old oil grease and antifreeze from around the hole and inside the hole.
Make your patch out of whatever, and test fit it, it doesn’t have to be perfect. Aside thought, but I used the Goodwill store like a parts supply. Sometimes I need a piece of plastic or metal of a certain complex shape, and I will go to the Goodwill store and find a toy or a frying pan, or a tin box or whatever that I could cut it out of. And then I saved the rest for the next project
When it’s dry, you can smear a little JB weld, E 6000/goop, regular old dollar store epoxy, whatever around the hole. Then smear the same thing inside your patch.
If it’s something thick, when you stick the patch on, it will probably just hold. You may have to put a big piece of masking tape to hold it on initially, but don’t pull the tape off too early, it will just pull the patch off.
After whatever cement has set, I would take some more and just spirit all over and around the outside of the patch.
Let me also say it probably took me longer to write all of that then it would take you to do it.
One final thought, when I am rigging something, which is a large percentage of my fabrication, I actually like to use a glue or adhesive or whatever that I can remove in case my rig doesn’t work out right. If you mess up with the JB weld, you’re stuck with it.
Get it, “stuck” with it?
My two cents, I hope it helps