I had to check out danny's thread again so I could refresh myself on what the frig looked like.
Sure I can help ya out, lets figure out the details of the box then we can figure out how soon you are wanting to get it going.
1/4" is probably a decent thickness if you want to keep it single wall. If you are able to keep it dry when not in use then you could maybe get by with 1/8" No water no rust to speak of. The ones that rust thru really bad are the ones that have ashes left in them and then hold moisture.
Looking at Danny's, I would think 24x24x24 would be plenty, probably even more then you need. I have never learned much about the frig builds so to those of you with frig conversions pls chime in.
With the frig being insulated I wouldn't think that it would need all that much fire to keep temps.
Square is probably the easiest to build and the vents he used are pretty easy to cut in. So thats not a big deal. The hinges are pretty inexpensive and I think I have a piece of scrap 1" round stock for the door handle.
Here is a pic of one of my fire boxes on my RF. I know it sounds weird but I have two fire boxes, I only use this one when its really cold and I am having trouble keeping heat. I run a race track in it and it works great. This may be something that could also work for yours. My box is 24 square and the low grate is about 4" above the ash.
The center area can be filled with water to help with the moisture factor. And yes thats rust LOL Some lame brain left ash in it and it didn't take long to start seeing rust everywhere.
Hopefully we get good input and suggestions on this.