Play in 11 speed cassette on Mavic wheel.

New wheels?
The Freehub could be buggered. Might need new bushing/bearings. Is the freehub marked with slits?
I'm in midst of a bit of a nightmare with an Elite rear wheel. The play had gotten to the pit of the gears missing changes.
It rues out I seem to have somehow managed to ovalise the axle.

According to some websites a few Mavic wheels in the early days were made with a freehub a few 10ths of a mm too long, and were originally supplied with a spacer. Maybe yours is one of those?

Local engineering firm could probably knock one up cheaper than buying a cycling specific one.

tbh I would just add a 1-2 mm spacer behind the cassette as long as all the cassette spacers (between the cogs) are included, its not rocket science, if you can get it to fit without rocking or binding, jobs a good-un.

I know Campag needs the very slim space on 11speed (Running Cosmics and Chorus 11) and a mate put the same spacer in on his DA11 on Cosmics from day one and has had no issues, I know mavic say it isn't needed though,

Stating the obvious a little, but tightened it up enough and got all the sprockets sat correctly? All spacers in?