joining copper sheet at corners

Rather than cutting a square out of the corner, you can cut the material so there is a flap under the corner that will make it quite easy to solder.

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You're not going to be getting any solder that is yellow or amber in color other than by actually welding it together. The best is to put the exposed joint at the corner and do a good job of putting the two pieces together so that the solder line is minimal. Any good solder will work well and the silver bearing solders will, of course, be stronger.

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I think one of the phosphor copper brazing alloys will be as close as you'll get, at least with common stuff. It'll take a higher temperature than solder, but a MAPP torch should do the trick. No flux is required.

The stay-silv and sil-fos alloys on this page are phosphor copper.

According to Ned Simmons :

But I suspect that you will have to form the corners over the wood, remove the wood, solder and clean, and then replace on the wood. I think that the soldering temperature would be high enough to ignite the wood. (Experiment with samples, of course.)

Good Luck, DoN.

You might cut the copper so it overlaps as suggested in another post, then solder it from the inside. The solder will wick between the sheets and if done right won't flow out past the ouside. It will be nearly invisible from the outside when put back on the wood.

Put a copper patina on standard lead-tin solder with a simple copper sulfate solution. If you don't have crystals handy to make some up, find premixed solution in any stained glass supply outlet. For instance: