Again, you can use inexpensive foam to insulate that gap. This time, you can buy the insulating foam tube that is used for hot water pipes. Get a small diameter for 1/2" pipes. Cuts easily with a razor knife. You can take a wedge out of it to turn a corner.
Should fill in that big gap nicely.
Also, you can get serious with silver aluminum tape (no, not duct tape). This tape is extremely adhesive, bright silver metal foil. You peel off the backing and it will permanently adhere to metal or even painted drywall. So, install the backer rod and/or foam tubing. Cover with silver tape.