Doc,
I have made only one. I did it years ago before ARL existed. It is on a 12 gauge smoothbored gun that is equipped with a hooked breech and keys because I thought, back then, that it was necessary to remove the barrel for cleaning.
This is how I made mine. I fitted the breech plug, 3/4-16 thread, just like you always do with a good snug fit at the bottom of the female threaded section of the breech. I the took a hack saw and cut off the tang. Using the hack saw again I sliced along side the vertical faces of the part of the breech plug that extend backward from the barrel, on each side, down to where the breech end of the barrel started. I then cut along side the breech end of the barrel to remove 2 rectangular slices. This left me with an extension of 1/2 inch in thickness, centered on the bore, extending towards the butt from the breech. I then cut the extension off at about 1/2 from the breech face. I cut down the top of the breech plug extension about 1/4 inch and filed the top surface to be a shallow V with one leg vertical and on the same plain as the breech and the other leg angling upwards as it extended towards the butt.
I took a piece of 3/16" X 1.5" mild, hot-rolled steel from the local hardware store and bent it with the aid of a propane torch, large vice and large hammer to a nice sharp right angle as the first step in making the soon-to-be standing breech. I bored a pair of 3/8" holes in the short leg of the soon-to-be standing breech where the hook extension was to pass and then with files opened up the hole until the standing breech fit the barrel. I flattened the standing breech with files until I had a nice snug fit. Thats when I discovered that it is necessary to round of or otherwise relieve the bottom of the breech extension.
With the standing breech fitting to the barrel I used hot-melt glue to stick the 2 together. Now I filed the standing breech vertical leg down to match the barrel profile. Once that was done I filed down the standing breech tang to match my idea of where I was headed and the stock profile. I then let the glued-together assembly in just like a regular barrel paying close attention to the fit at the rear face of the standing breech/stock interface.
The gun, though ugly, has shot many things over the past 20 years.
You can do it!
Best Regards,
JMC