I built several locks from scratch in high school many, many years ago. I have never had the urged to even assemble one since (although I have done so from Blackley and TRS parts). I'm with Barbie and Jim on this one.....just as in the 18th century, I am content to let lock makers make locks and I will enjoy the fruits of their expertise and labor.
Here is the story about my first locks........
In 1969 I was a sophomore in high school. I had a Dixie catalog with a 2" x 3" very poor black and white photo of a flintlock rifle. You can imagine how much detail there was. At the time, this was the best lock I could come up with.......
I built one rifle with one and it worked OK, but I innately knew it that its architecture looked more like a canoe paddle than a rifle...and I knew the lock was junk. I determined to build a better lock (from looking at a bigger picture in the Dixie catalog). I made a set of master parts in silver, brass, copper...anything that was easy to work. Then I vulcanized a set of rubber molds and pumped waxes of all the parts.....
I found a local foundry that did steel castings. The guy who owned the company got a kick out of a young kid trying to build flintlocks and he said he would cast as many wax patterns for me as I wanted to make up for $25. I didn't have $25 but told him I could borrow it and he held out his hand and said "Deal !" I made up 20 sets of wax parts and brought them back to him and he cast them all for me in a few days. However, when I went to pick them up, he was holding a box and he said that the castings didn't come out as good as they should have, so I could have this batch for nothing. When I opened the box, the parts all looked fantastic to me. When I looked back up at him, he just winked at me. I eventually had him do some additional castings for me and insisted that he let me pay for the next batch, which he did.
I started putting the pieces together and making a lock plate out of a steel strap......
Finally got the lock all together and hardened all of it.....and then, since I couldn't engrave, I covered the plate with asphaltum varnish, free hand scribed the decoration I wanted, etched the pattern with dilute nitric acid, and then gold plated the etched lines before I removed the asphaltum mask.
I built two locks and then made a matched set of rifles for my Dad and I. The barrels were Douglas, 13/16 inch straight, .45 caliber. Not great locks, not good architecture on the rifles, (but much better than the first rifle with the Spanish lock.) We still have the pair of rifles and they mean a great deal to me for the all the lessons I learned building them at the tender age of 16.