I'd seen that coupler, it goes between the shaft of a small (really a baby) supercharger and the pulley where the belt drives it.
My supercharger is MUCH bigger (read more torque required) and does not have a belt, the main shaft of my supercharger is lined up with the crankshaft of the engine (low and in the front) so I need a straight through coupler. The engine will be mounted on rubber bushings, the supercharger will be mounted to the engine, with a rubber mounted support on the rear. The overall length of the setup, from rearmost engine mount to supercharger is 7 feet, with rubber mounts on both ends and the front of the motor (~4ft from the back) I don't think I have the space (or the weight) to rigid mount the supercharger. So a slight angular and linear misalignment may occur under heavy load or acceleration.
The conventional pair of universal joints would be great, but not enough space for the ones that can handle the load, so I need something a bit more elegant. Some of the earlier posts have good ideas that I'll build as a prototype sometime soon once I get back to the shop.