Lunar ecclipse has slowed down the internetz
Kncool,
I'm unable to pull up pix, try to chime in about using one of those heavy duty Wald rear basket arrangements to stablize that part of the puzzle.
They have large twin baskets 18x7x2.5", and rear grocery baskets approx. 13x7x8.5". These could be modified/cut/hinged to provide the cradle that holds the bulk of the boards weight.
Then the board could either be attached similar to your pix OR a "sidewise skateboard" dealio could be built, with independently spinning dolly wheels to pull behind the bike.
Also, if the sidewise/skateboard idea is envisioned, instead of basket(s) on the rear, a rack arrangement, where the board lies flat behind the seat with 2 or 4 wheels keeping the board off the pavement would be more "wind freindly"....but the wheels have to be dolly type, not paired on an axle, to make turning easier.