I used a Comair MX2A4 115cfm A/C fan for this project. I bought it at Grainger (was about $30).
I took a few scraps of 1/4 sheeting I had laying around and popped a 4" hole in each with a hole saw. Most any piece of scrap wood works for this - it doesn't have to be 1/4 inch.
Get two 4" tab collars. Tab collars are intended to be used on material ~3/4" in thickness - use a pair of tin snips to extend the tab cuts. See tab1.jpg and tab2.jpg for before/after shots. Note that the tab collar in the photos is a 6" version - I didn't have another 4" laying around.
Place the tab collar in the 4" hole you drilled in step 1 and fold the tabs back. Repeat for second tab collar. Use a pair of leather work gloves when folding those tabs back - once you cut those tabs with the snips, they become *real* sharp.
Stack the two tab collars and the fan in the final position, then using your drill and 3" sheet rock screws tighten things up. The sheet rock screws will go through that tab collar like butter, so make sure your finger isn't lined up with that screw when you put it in!.
Attach the inline fan assembly to your light and exhaust ports. Note: I had previously modified that light hood as outlined in my sig.
All of these fans require you to buy a "pigtail". See "Optional Accessories" below item on each page. Avail in diff lengths and 45 or 90 degree heads, but they really aren't optional. They avg about $1.50.