You will most likely get some feedback here, but you might also try the QAJAQUSA board where Tom Yost hangs out. He can give advice on scaling it down.
http://www.qajaqusa.org/cgi-bin/GreenlandTechniqueForum_config.pl

Sea Flea's are so light that you could just bungee it next or on top of your tandem. Maybe make a couple foam pieces to help things out.