Most small systems will only function correctly with one port open at a time. In some cases, two ports work.
By small systems I mean 2HP collectors and yes they usually have one 5” inlet.
It would be best to use 5” duct, but that is not something easily found, and expensive if you do find it..
By two ports I mean a 4” and a 2 1/2” open at the same time. A setup used on router tables and table saws sometimes; 4” to the saw base and 2 1/2” to the table top blade guard for example.
I think what you refer to with the “sheet of metal in a slot in the port” is a homemade version of a blast gate.
Sheet of metal in a slot will have leakage that will rob the system of performance. Buy or make true blast gates for this function.
-- Michael :-{| Diapers and politicians both need to be changed often; and for the same reason.
