You can use red RTV silicone as long as you put a large/healthy/good bead on the ventor and the collector box and make sure it makes GOOD contact/oozes out a bit. Then let it cure for 8 hrs or more B4 you fire it up or it will ruin it. The gasket is a better choice and some heating suppliers will sell fake asbestos gasket material to make one from or try get it from Trane.
Yuri- I contacted a local heating supplier... I can purchase a sheet of fiberglass gasket material, about 1/8" thick. I will try making a gasket out of this material. Any suggestions? Thanks for your help!
Put it over the hole on the collector box and trace it out by tapping it lightly with a ball peen hammer to make an imprint. Or poke small holes to make the cutout circle. Put a couple dabs of RTV silicone on the collector box to hold the gasket in place while fastening the ventor fan on.