First question....
I did use a plastic fume chamber and I let the copper sit over night, roughly 10-12 hours then I merely washed off any excess, waited for the surface to dry thoroughly and sprayed with a clear indoor/outdoor Krylon spray. In adding patina, you have to worry very little about surface scratches, the patina erases them. The biggest issue was cutting square shapes in 1/8 inch copper plate. I know it is supposed to be a soft metal but when you have this thickness, it does not seem so soft lol. The top plate weighs 2 lbs and does not move or bend in the least compared to say aluminum of a lesser thickness. I achieved more of a blue finish with ammonia, with vinegar I believe you get more of a green hue. My second copper build will be polished copper and trust me it will take much longer to buff out all scratches.
second.....
Basically for this project I bought most everything on ebay. 1/8 inch copper plates, 6x10 and 4x4 inches ($57) per set and thus far I have three sets. I did not have to cut any copper plate to size, my exact sizes needed were on Ebay (Metalremnantsinc)
Same with all copper and brass parts. If you search ebay carefully you will find people selling real brass watch parts in a group for very little money. By dumb luck, a vent hole I drilled in the top plate was the perfect size for brass pocket watch parts I found. I also found half inch copper pipe sold as a group, elbows, T's etc. I used those to build a matching headphone rack.
I ordered VU meters from ebay as well as Amazon, they are all the same size, if you watch carefully you can get a set with circuit board for $20. They are 100 to 500 microamp deflection but I had to use current limiting resistors to get proper deflection (around 16k resistors as I recall). I am using dual Alps pots and trust me on some music it is wonderful to turn one channel or the other up a bit more than the other. I used the output to the headphone jack as a source for the meters. As I recalled from some of my electronics theory, normally on a speaker system they are in the last stage before the speakers thus the headphone jack is as close as I can get to that as possible. (headphones being basically tiny speakers)
I may show the wiring when I do a more professional build, I rushed through this adding molex connectors and such so I could easily unplug all plates from one another. I am also going to install a speedball upgrade, I have two working amps so I could make certain that none of my changes hurt the overall sound of the amp. Because of the dual volume controls, I totally prefer the modified build so far.
The headphones I am using are modified Beyerdynamics, I added drivers to a set of California headphone company silverado's and turned them from closed to open. The other set is from Darth Beyer, modified DT 770's I believe that are more semi open and are built for bass in my humble opinion. I built all cables and made a set to match using some silver plated braid and ofc center conductor. Others within the setup are Mogami star quad wired as balanced using the shield as it was meant to be used in my opinion.
I apologize if some of this info is redundant.
Tom Stanley (I believe I am supposed to list any company related affiliation and it is included in previous posts.)