You can get a cheap, throw away grade case and cut it up as needed. Many of them have removable motherboard trays now. I had one that actually had the tray and the card support as one removable chunk which would work perfect for something like this. You might consider going to Fry's or a local PC store to see if they have anything super cheap that would work.
I actually forgot that I bought a plastic (yes plastic!) case on clearance for $10 a few years ago. It's just sitting in the box collecting dust. Though the case is plastic, I think the frame, tray, etc. are all metal.
The wood idea is an interesting thought. That would probably be much easier to work with and to mount than metal.
I was weary on "the pen method" due to gravity. But, the way I am mounting my MB, the card would be sticking straight up, so there shouldn't be much strain on the card. If it holds with the card sideways like in those pics, it should easily work in my situation.
I need to look at that old case and do some measurements. Seems like I have a few options though.