Sorry I’ve been away from the forum for a while, as, unfortunately, I have had to leave this project alone for personal reasons. However I have started up again. As stated by others, trying to solder joints in such a large chunk of copper is a bit bothersome (polite version) but I have succeeded with two of the joints. The third is not quite doing the job and the last just won’t do it. I had a thought and I need to ask those with more knowledge than me about RFI Electromagnetic Shielding Copper Mesh. I bought a square foot of this and have tried two ways of sealing the joints. No 1 Is by using the mesh as an anchor in order to spread solder over it in order to make an RF leakproof joint. No.2, and this is where I need the advice, was to glue two pieces of mesh, at 90° to each other, over the joints. Would such a joint br RF proof, especially at the frequencies we’re discussing ?. if not I’ll fall back to no.1 and also use that for the lid, as and when I get that far
@A342
Welcome back! I was only thinking about you at the weekend when I moved one of my home brew filters.
Looking good 
I can’t answer your question about whether the corners will be RF proof but they look good to me.
Good luck with the next part.
A question for the experts.
How do these things work?
I don’t think you need to spread all the solder like that. Just solder the edges and pour some epoxy (A + B resin) over it to hold it in place.
BTW, you can’t solder the interior joint only? Propane torch, colophny rosin…