[idontknow]

The pressurized drain line is most likely from a lack of a drain line trap. A simple U trap should work to keep the air from blowing out but the water flowing.

I see you have a sort of access panel on the duct box, what does that connect? The one side is the evaporator coil pack is all I can tell. Next time up, buy some foil tape and open the access door. Have a peek around looking for gaps. If you shine the flashlight on the outside while looking in it should show you any cracks. Make sure the area is cleanish and use the foil tape to fill in all cracks and openings. Then coat the whole area as best you can with the goop. Goop the area inside and out.

To help seal the area around where the drain line connects to the coil pack, find one of those swimming noodles?. Piece, cut and fit it so it creates a larger sealing area around the box. Electrical tape holds the sliced parts together and goop it all lightly but completely.

Swimming noodle is a soft foam hollow noodle about 2.5" round and 5' long. Kmart and Walmart have em in the pool section.