Use your method really works greats and easy to apply thank you 😁I have always used pipe \ duct sealant. It looks like a block of black chewing gum. Electricians and plumbers use it to seal up pipes when they drill thru walls. It won't shrink, or rot. Sealing up the airbox will help the fan blow thru the vents better was well
Gardner Bender 1 lb. Plug Duct Seal Compound DS-110 - The Home Depot
Gardner Bender Duct Seal Compound Plugs is a dough-like material that is easy to handle. This material helps seal and protect gaps, holes and conduit openings against dust, moisture, drafts and noise.www.homedepot.com