Heater duct; what tape to use?

[Crazy-J]

Hi all,

I recently installed a heater/defroster from vintage air (just a heater, no air conditioning).

Because of the difference in size of the duct coming out of the heater to the larger duct needed for the defroster vents I am using two different sizes and I need a way to keep them together where one hose ends and the other begins.

I tried duct tape but it just burned right through. I next tried some heater hose wrap tape that worked for a little bit and didn't melt but it won't stay stuck to the ducting.

Any idea on what tape to use?