I could design and build the world's smallest, er, briefcase or something equally pointless, but why would I? Away from the hands of the experts, the soprano sounds bad enough! The sopranino has no purpose apart from when it is in the hands of the most exquisite player.
The soprillo is an extreme artistic experiment and serves no useful musical purpose at all as the frequencies achieved can be much more elegantly acquired using a piccolo or a cheap penny whistle.
Really, it's like making an electric guitar that's only an inch long -- you can do it but it would be crap and no one would want one.
Regards
Martin -- a confirmed low-frequency fan