since the switches, scavenged from a cherry keyboard already had diodes installed, all that was needed was a small wire bridge - made with a small bending jig from solid-core wire

... the same wire which is used to connect the rows...

... and columns.

this picture shows two modules in action: an I2C_secondary_bridge in the bottom-shell, and a matrix_mcp23017 connecting to the key-matrix, with the red expansion header going to the thumb-cluster layer on

the thumb cluster is basically one row, which is run from switch to switch, and six columns, run to each switch separately