They were cold formed.
As a flat bucking surface - a hand held, chunk of steel with a flat smooth face (definitely nothing larger than fist sized needed here). For the head (preformed, round) end of the rivet, I happened to have a suitably shaped set to fit an air powered rivet gun. The main difference between a rivet gun and an air hammer is in the trigger - rivet guns have a teasing trigger that makes it easier to get the gun to give only a hit or two at a time if you want. That said, you should be able to get by with an air hammer or a manual hammer for the few rivets involved.... to build confidence do a couple as practice in a couple of pieces of sheet you don't care about that match the thicknesses involved. For the few rivets involved a used rivet set should be adequate - or borrow one from a friend (or you might become the one everyone wants to borrow from...). Most rivet supply houses also sell the setting tools. If you happen to know someone who's into homebuilt aircraft, they might be willing to loan a rivet gun or use their's on your project.
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