Oops, didn't realise the intended recipient was one of those.

I'll go one better than filiing down the wide spline on the cogs and using a Uniglide top cog (that last is less than ideal) - you can probably* disassemble the freehub bodies (with a suitably-sized bit of metal plate) and exchange the cassette body's shell.

It might require a bit of modification; I HG'd an old Dura-Ace AX hub that had a riveted body, but I had tp grind down the back of the shell to fit inside a lip on the flange. Also, a spacer was required because the splines went too close to the spokes, but it allows a proper HG top cog.

*Not sure about the screw-on bodies; they might use a different system, or you might be able to secure the threaded portion in a vice (with some aluminium to protect the threads).