I would use a 9-speed cassette with the loose cogs. The smallest cog has a built-in spacer; this would be too thin for 8-speed spacing, so would have to be augmented with a little more thickness. About 0.5mm to be precise. I would cut a couple of spacers out of a alu pop can. My guess is the can thickness is around 0.2mm.

As far as the other spacers, since Shimano 7-speed has the same cog spacing as Campy 8-speed, I would try these. I would go down to my local bike Co-op, and fish out a discarded 7-speed cassette to then crack it apart to get at the spacers.