I get your point. I guess this would be the long way of saying 'because of the way I intend to mount them'?
120 (4.72" - say 4 3/4) was too large. I went for those first and realized they won't work. I have 3.75in to play with. It's hard to tell from the pic because of the angle, but 3.75 is my max.
80mm is roughly 3". that gives me roughly 3/8 on each side to run a rail (mount points) and straddle the length of the cavity or cut holes in a solid piece of wood. I attached a few rough drafts of what the mount may look like. One uses spars, the other a solid piece of wood. Both approaches would simply use 'L' brackets at each end for support.
I'm beginning to favor the latter because that would effectively 'seal' the boundaries a little better, forcing the air up through the stack. Some may still get pulled downward through the space between the bottom coils and back of the fridge and my hope is it draws more so through the vents (cooler air).
My plan was to mount the fans at the top of the opening in the pic to 1) maximize the number of vents (3) and 2) possibly create an upward 'draft' effect once the fans spun up to take some of the pressure off the fans - using the old heat 'rises' mentality. I figure once I got things going, mother nature would assist the drafting process. It could also help the fan from blowing pressure out one or more of the lower vents rather than past the coils, as intended.
If you see anything glaringly wrong with this approach, please don't hesitate to speak up. I value people's opinions, even if I don't always adopt them for myself.
While we're on the subject, does anyone know a good place to tap 12vdc other than the 12v going to the board?