What you are probably seeing is the pipe sweating, the suction line would be cold, and condense. If you cut a piece of sheet metal in half, and install it from the inside, it will naturally be held against the furnace. A little mastic or perhaps duct seal around the pipe may work. It should be dry when you apply it.
This is a very good idea, however I would add cutting half circles in two sheet metal pieces positioned to line up with the big pipe and the smaller line. Over lap them and they will enclose the copper line going into the plenum.
If there is enough clearance to secure the new sheet metal with screws with out getting too close to copper lines and run the chance of perforating them,
You might try riveting the metal with a short drill.
Before finish enclosing the hole, check to make sure the small tubes off the small line inside the plenum aren't rubbing. If the are chaffing they can cause a leak.
Thum gum is like a piece of clay or a very dense play dough.It won't be any good for holding that small piece of sheetmetal to the metal plenum,but it will be able to fill the small gap between the copper lines and that small piece of metal.That is why If I did it I would use screws to hold that small piece of sheetmetal to the plenum.
I agree. I didn't mean for the duct seal to be used without mechanical fasteners.....although when I read my post is sure sounds like it.