yes, you can make a thief from plumbing parts, and it will work, but is ugly.
also all plumbing fittings are designed to slip over the outside of the pipe for soldering, with 1.25" pipe this makes it too big to fit into some barrels bungs. so you need to make it out of smaller pipe, and you can draw a much smaller sample.
while it solders together in much the same way one would out of plumbing fittings, I've designed my pieces to slip on the inside of a standard 1.25 "L" pipe length, making for a tight, flush fit solder, without increasing the diameter. Custom machining of copper isn't cheap, even done en masse, but you have to have a bunch of pieces made at once to make the price approachable.
I did this more as a favor to my customers, I don't make much on these thiefs, I just thought I could do a better job of it than what was out there. Doesn't hurt my feelings if you don't buy one.
I tend to give these away more often than I sell them. Typically to Customers who have bought a still from me, or Friends in the industry.