How do I block a furnace outlet at the manifold?

I'm not blocking any output from the furnace. I'm rerouting one output from the furnace and closing off one inlet to the receiving "PLENUM".

Before I posted, I wondered what the correct terminology was and googled both terms. Obviously, I saw the wrong description for plenum.

Good feedback! Thanks!

I agree with not blocking it, just add the new duct and try it out. The under floor duct may also keep tanks and such warm, and if it’s blocked right now there’s really no advantage.

he said he will reroute the disconnected outlet to a new vent he should be fine.
4 in. Round Duct Cap

Having had a 19T4, I remember (?) that the furnace had metal blanks that were knocked out to attach the outlet flanges. A piece of thin metal could be sized to fit over the opening and screwed or riveted on to block one outlet. ??

As to blocking and overheating, the furnace has a hi-temp cutoff switch that would prevent that from happening. And if you open up another outlet, the air flow should be the same or nearly. If the floor duct is already restricting the flow, obviously the system is still working!!! Had one trailer that "operated" on the cutoff switch when heater running!! Ran that way for years before getting another trailer!!

It is called a plenum, not a manifold!
I would not plug anything off because it cause the furnace to over heat.
Odds are the duct has collapsed. Yes, it can be difficult to gain access.
Make and model of furnace would or may help. The installation manual may show how toremove furnace to get to the duct through that access.

If you are just removing one of the other "furnace vent outlet covers", use the new one to cover the old one (at least on the furnaces I have worked on).

Worse case is buy a small sheet of aluminum from any box store, drill holes at the corners and screw it over the old hole. I would not trust any glued product in that high heat environment.