We got standard open frames in good quality (aluminium) including pre-screwed holes, screws and spacers in Switzerland incl. 6 Risers for less than CHF 300,-- so i think you should think about your calculation again.
A closed frame just raises the noise level as you need faster fans to get out the heat from such a small box.
yes, we now realised we are totally off the price range. We have to recalculate and see if we still want to do this at the given prices.
As for the closed design, we have to prove you wrong. This design is sucking air through the casing and is working very well. There is a tower of 9 casings running inside a closed wooden frame. At the backend of this frame is a industrial fan attached sucking the hot air out of the room. If someone is interested I could put some pictures only of this setup.
With this setup your heat is not spreading, you can control where the heat is going and deal with it.
We didn't want to go with threads in the aluminium so we used threaded rivets. This way we can easily fix damaged threads if a customer would hit a screw to hard.