There's a SUP guy or two with a bad back.... these systems would help them too.
Rola probably have something similar. Rola have been good to SUP people... sponsoring events, etc...
You could make a fixed system similar Thule Slip Stream, with long sticks of 20+mm Dowel, and boy scout lashing, with rope... and a cross bar... still using lashings... No bolts, no screws... just a hand saw to cut it to length.... It would cost about $15-$20.... It would look silly, but it would work!