Simon, unless you have your heart set on real single-slots, know that several early cars are running around today with double-slot wheels in which the lower slot was carefully filled up with J-B Weld to give the appearance of a single-slot. I know of one car with '10" single-slots' that were apparently never made by Campagnolo. One owner said he only expected a few months before the filler cracked & fell out, but his original work is now on its second decade without any sign of cracking. Obviously, J-B is not a permanent change, either.
A real single slot cut-out is actually positioned in between the two cut-outs in a double-slot, but single slots are so scarce that almost no one will notice the slight misposition, especially if you then repaint the whole wheel with Argent Silver. And any spare wheel you might find needs baking-in-Mom's-home-oven at 275F for a few hours to stress- relieve it every 10 years or so. I'd do this before J-B-ing & painting as any filler will heat-shrink and the paint discolors.