SAAS make a cast aluminium 6 bolt boss that is a good bassis for any wheel. Autotecnica make a boss too but it's a mix of steel bracket and aluminium base so not as adaptable.
I liked the look of the 9 bolt Motolita wheel so I machined down the SAAS boss and machined up an adaptor ring to step it up. I also machined up my own horn button assembly for a cleaner look.
Not a very good picture but you'll get the idea.
Cheers