It is not DIY, but you have an isomorphic keyboard on the Dualo.
I have one, it’s a clever/modern tool !
It integrates also a nice midi looper wich make it easy to make midi loops.
see video tutorials at https://dualo.org/community/videos/#tutorials
It outputs midi (trough its midi or usb conector) on up to 16 midi chanels , depending on the selected midi mode. (see pages 28-30 of the user manual https://dualo.org/support/# )