Aluminum foil duct tape
I routinely remove my heater during the summer, actually, now that I’m in Florida I tend to have it off about 9 months of the year. I seal the hole in the baffle with 2 layers of aluminum foil tape. The same stuff that is used on HVAC systems. One layer of tape would probably do the job.
I sticks well and comes off fairly easily when I want to re-install the heater.