Wahoo Kickr with a 10 speed cassette but my bike is running Shimano 11 speed, just fit an 11 speed cassette? Picked up the Kickr 2nd hand without thinking about the cassette etc. details.  Not too fussed about having all 11 ratios on the trainer so wondering if installing a spacer or similar will work or do I need (and can I) to fit an 11 spd cassette to the wahoo?
Presumably the Kickr has a '10 speed' 34.95mm freehub body rather than a '11 speed' 36.75mm one.

Some wider-range 11 speed cassettes come with a 1.8mm spacer and will in fact fit on a 10 speed freehub body. The larger sprockets allows the spider to overhang the inboard end of the freehub body without the sprockets interfering with the spokes (which you of course don't have on the trainer).