My uncle built one but it was an old wood sided one. I can post pics if you want.
I think would have to drill a lot of holes in the top of the metal sides and bolt your 2x4 down to it to start a good frame work. The angles on the open end will make it harder to close in but it could be done.
If your going to run a heater in it I would go with double pain windows or you will have a ton of fogging issues.
pete said:Let me know if you do anything. I have a wagon just like that one sitting in the bone pile. Missed you at the classic this year, we need to keep in better touch!
Lets brain storm those wagon blind