I was wondering about a velocity stack made of some flexible polymeric material that could be easily slipped over the carb inlet & could also be easily cut to to adjust the length. Kind of like the silicone rubber exhaust extensions. We would need a removable bell-mouth at the inlet.

I think that would be much easier than mounting the carb on a long inlet runner & trying to cut that back in steps (throttle linkage would be troublesome that way).

The elbow (metal) idea could be used to "tip" the carb up vertical, so that the polymeric stack wouldn't get too close to the prop.

Like a tuned pipe, this would be RPM specific, but I think that it could be quite effective, particularly if a synergistic effect could be tuned in with an exhaust pipe. Tricky stuff.