0.2mm washers would be thin, but you can probably find them somewhere. You can measure your freehub, and don't really need notches.

I'd go with regular spacers wherever they would work, but perhaps use the thin washers in places that had built in spacers. I don't know if spacing out the first couple of sprockets would be good, but perhaps it would be OK to use stock spacing between the 11T and 12T, but add a spacer between 12T and 13T (or whatever you are using).

I believe that in general, Dura Ace and Ultegra level cassettes use 2 common carriers.
105 level cassettes use 1 common carrier.
Sora/Tiagara, and lower quality cassettes don't use common carriers.

Also consider, if this is mainly be used on a trainer, you might not actually be using all 10x2 speeds.