Paul Everett has created a MIDI keyboard with buttons laid out like a Hayden Duet: MIDI Gadget. It’s nothing new really since this first appeared online in 2003, but somehow it resurfaced and it’s cool enough to keep from you.
Hayden Duet concertinas are simple, logical, and easy to learn. They are reputed to excel for playing by ear, chording behind a melody, and accompanying singing.
Also check Wim Wakker‘s website. He has been making MIDI Anglo concertinas for a while now.