I'm trying to find ideas for repairs on my air unit. After going under my house to fix a water pipe, I notice my air duct is no longer attached to the unit (It was literally duct-taped together. -_-)
The problem I'm coming into is that the ducts appear to connect to some sort of fiber glass box. The box is the same size as the air unit it connects to (I guess that's why whoever put it there thought duct-tape would work) I clamp obviously doesn't as I have no space to clamp... There is some form of dried plaster on the other side that also appears to have duct-tape under it as well.
Any suggestions on how to fix this?

Basically, the connection looks something like this... [ ][ ]
I know I need to get some sort of support under both pieces, just unsure what I need to get to keep the two connected. They're identical in size...