I started by welding a small square of angle together. I felt really proud when I'd done it. Then I noticed how warped my creation was and then how one angle wasn't right and I couldn't even fit the CPU fan the box was designed for. So, out came the angle grinder and version 2 took shape. Now the reason it was off kilter could have been due to the angle maker I bought from Harbor Freight. While it gets the 90 degrees right, the horizontal hold can slip a little.
Above, you can see the fan holder beginning to take shape. The plan is to have the fan at the top and the filter on the front of my ventilation unit. It's done this way to try to keep water out of the mechanism. The next challenge will be a vent and filter changing door. For that I'm leaning toward a removable door that presses the filter into place and just screws on.
That's the unit so far. I was wondering how to attach the fan. I'd been thinking of a spring clamp. Then somebody suggested making the bottom removable and using ordinary screws. Well, I could do that but M5 is way too big for the holes in the fan. 10-24 is also too big. I'll have to try smaller bolts. The best solution would have been roof vents but as my bus was not equipped with emergency escape hatches. Thus I have to do what I can. This might not be the full solution but it should help. The white roof and the heat extraction fans have already helped. If necessary, I can always upgrade the extraction fans to something more powerful. For $30 each there are CPU fans that claim 200 cubic feet per minute. My bus interior is 24 feet by 6 feet by 7 feet approximately or around 1,000 cubic feet though it'll be less because of stuff inside. That'd take two fans at a combined 400 CFM around 150 seconds to expell all the air in the bus. For the mention though, I'll go with what I have planned.