Hey GreenShamen
I couldnt get Key-Cad to work,so heres a drawing I just made.It's a cross section of the vent.You can build it to the size you want.Paint all the peices
back and caulk all the jonit.I have a 3"x3" muffle fan that puts out 25 CFM'S
mounted to the bottom vent blowing into the closet.Hope you can use it.