Ours is 37 cubic' and it has a 5" vent at the top, the vent goes out through the wall and is terminated into a tee with a Breidert style cap. The vent is double wall B-vent pipe and is also insulated with high temp insulation. The vent has a very good sealing damper right where it exits the oven. That allows us to regulate the amount vent and retain heat when possible.
The air inlet is on the very bottom of the oven floor, there are a series of 2" holes covering the floor under the heating element.
You could probably accomplish the same thing with another 5" pipe near the bottom... and add a damper to help regulate the inlet air.
Our shop is 60 x 66 x 16, before we vented the oven to the great outdoors larger parts being cured would drive us out. That is with the hanger door open which is 45' wide x 14' high. Basically a wall removed...cough, cough
If you mount the fan OUTSIDE of the oven and run the shaft through the wall you can add a blade inside the oven. Be sure to add a blade outside the oven as well. The second blade acts as a heat sling and keeps the heat away from the motor. Make sure and blade shrouds... those blades make an excellent meat slicer as well..
IMHO