02-14-2009, 11:42 PM
You could always form a flange on that piece of ductwork shown in the last picture. On the large end, mark a line around the outer circumference, about 1" in from the end. Using tin snips, make cuts from the end to the line, spaced 1" or 1/2" apart, whatever you think would work best. Use pliers to bend them out 90 degrees to the circumference. Select a few spaced evenly around, then drill them to accept pop rivets - use this as a pattern to also drill the plywood around the fan outlet. Spread a bead of caulking on the flange, then press the duct into place and install the pop rivets. If you think that you'll need to disassemble this at some time, you could substitute screws for the rivets.
Wayne
Wayne