Look around for some stiff plastic sheet that is 0.2mm thick. A few years ago I used those opaque plastic milk cartons to make some spacers to adjust a shimano 9spd cassette to Campy spacing (i think it was 0.3mm). Try the clear plastic film used in photo copiers for example.You need to cut them in a ring and scissors work OK. Inner diam fits over the spline and about 5mm more to outer diameter.
And I agree with CliffordK - use a cheap cassette as on 105/Ultegra/durace the large cogs are mounted to a separate aluminum carrier that is one piece so you cannot put spacers between the cogs.