[chromo]

You don't really want the fan in the oven as is. I dought the motor could take the heat like that. And you don't want a hard wind in there either hitting the parts, might blow off the powder.
Normally I would think a duct down a back corner, maybe just a an L shaped metal sheet screwed to the corner. That would probably cause problems for your shelves though. In this case I might use a furnace type duct pipe of maybe 3" Dia, run it down the outside and use an elbow (90) at top and bottom, insulate it well. You want the fan motor out of the heat so on the outside, the fan blade would be in the pipe. Might be able to get that blade off and make a shaft extender for it, or find a slow motor with a long shaft for it.

As for removing foam if you have it, I am not sure the best way, but what I would do/try myself.
Cut the outside panels and leave about 1" or so at all edges, you would basically have a outer frame work left, like an angle iron frame, but only sheet metal. Then you need to clean out the foam in those corners also. Some of these it may be that the foam is the only thing holding them together and if so then as you clean a corner out add some support to hold the liner in place.

Years ago I had animals, hogs, goats, etc.. and something was killing them, I had to dispose of dead ones sometimes. I used a chest freezer for an incinerator to burn them. It was a foam type, after I burned it out I had inner outer shells and the inner just lifted out, nothing but the foam held it together. Don't burn it out though, it ruins everything of course!
That was the first foam type I ran across, and if I knew it was the foam I would not have burned it, but I expected fiberglass inside and that would have been fine.
Later I found and killed the dog that was killing my livestock, was last time I had to use that freezer!

Well on the shop vac, I took it back and got a refund. Bought a larger hopefully better one.
The one that fried was only a few weeks old, maybe 4-5 weeks when I looked at the receipt. Not used much.
I don't know why it fried really, but the cloth filter was really caked with dust. Maybe it was was overloading the motor as if clogged? I was getting a decent suction from it until it smoked so who knows? Hopefully this new one will last longer.