Yup, you can make them look exactly like a cupped collar, and a domed collar will spread out the peening force over a larger area of the scale to the outer surface of the collar.
A domed collar will conform to the shape of a curved scale and the edges will be flush to the scale.
Additionally you can tailor the size, (dimension) to match any collar, by turning down the size of the washer easily buy bolting it onto a mandrel, (nut, bolt and spacer tube) and chucking in a drill or drill press and holding a file to the edge to the desired size, as demonstrated in the polishing sequence of the post, linked to in the post. Instead of polish use a file or sandpaper.
Years ago, Voidmonster handmade (no lathe) a die from a steel rod, chucked in a drill press, turned and formed triple stacked collars, from brass sheet. The collars came out very well. I’m pretty sure he posted about his process.
This was before AJ started selling his stacked washers. I’m sure you could do the same with small cupped collars, but why. Domed washers look the same and work fine.
AJ makes nice stuff, can’t go wrong with his collars. I use his triple stacked collars, small ones, I dome small flat washers, made tons of them, buy flat washers from Micro Fasteners.