@bender @movq Thank you! Not sure what I end up putting in there, but I'm sure I will find some tools to go in. :-)

Yes, this was a flat piece of sheet metal. It went together like a cardboard box, just much slower and with timbers clamped down to get a straight folding line. I don't have a sheet metal brake, so I just carefully hammered the piece bit by bit. Like in this video by the Sheet Metal Dude: https://www.youtube.com/watch?v=WYgEfWEMXk0