S and S makes some couplers for small diameter tubes,(down to 5/8")
as well as for main triangle tubes.
a frame builder that does retrofits can fix your bike,
and has the account with S and S for the parts.
http://www.sandsmachine.com/

has a current builders list.. many .

Bike Friday has made an opening with a hand shake like pair
of machined steel pieces that bolt to each other .

a straight forward job for a small machine shop,
doesn't even have to be a frame builder.
seek out a local machine shop.