Hate to break your bubble,,,but as a 2part epoxy (plastic), JB weld is gonna melt/fall out soon as heat builds. I use a product at my shop (custom exhaust,muffler shop a specialty) called "Muffler Cement" that will fill and STAY in gaps up to 3/8". Have no idea if it's available anywhere but exhaust specialty suppliers. If you like I'll try to get company name,address, contact number first of the week. Second choice would be old fashion stove and furnace cement, as used to repair/fill gaps in woodburning stoves,furnaces, and fireplaces. This product is available locally in this area at a mom and pop hardware store. Have never noticed it in the HD-Lowes stores,,,,but then again, I've never looked for it there. This is kinda like a gritty-sandy filler that is extremely brittle after hardening, but it will also fill gaps reasonably well untill it's taken apart again.
Bob