Spacer for 10 speed cassette on 11 speed freehub

Hi due to my issues yesterday with my spoke breaking, I am trying to put my 10 speed Ultegra cassette onto my 11 speed 105 freehub on my unused winter bike wheel (the 105 freehub wheel).
However, when I try to do this and tighten up the lockring, the Ultegra 10 speed cassette is loose on the 11 speed hub (obvious when I think about it but I thought the 11 speed would be thinner and so the entire cassette the same as the 10 speed). So I need a spacer right guys???? Where does that spacer go, on the inside or on the outside near the lockring please? Can I just use another Shimano spacer from another cassette?
Many thanks!
However, when I try to do this and tighten up the lockring, the Ultegra 10 speed cassette is loose on the 11 speed hub (obvious when I think about it but I thought the 11 speed would be thinner and so the entire cassette the same as the 10 speed). So I need a spacer right guys???? Where does that spacer go, on the inside or on the outside near the lockring please? Can I just use another Shimano spacer from another cassette?
Many thanks!
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they come with every cassette so they should have a box of them from old cassettes they have taken off.
You may be lucky and get the 1.85 to work OK but it's possible an extra 1mm one if your existing 10 speed cassette does not already have one.