There are probably more than a couple that would fit, but mounting them (securely) is where some design adjustments would be needed.
Someone here (maybe Hizzo?) mentioned trying to use magnets to hold the seat inplace.
And others have chopped the seat into a solo one, or atleast the passenger backrest cut down, and somehow mounted it, so there's always a way, just that ya got to figure it out.
Doesn't someone here (or maybe that's at Y!) have a Hardley solo seat on their Vulcan? How's that mounted whoever ya are??
AKA: Tim & 'The Adventure Cycle' VROC #24567, NEVROC, SteelCity VROC
"When life throws you curves,
Aim for the apex."