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Does anyone know of a decent flight case for a Neptune (Rotary if it makes a difference) that will bring the whole thing in under 70 pounds? I seem to doubt such a thing is possible but I figured I'd ask.

I built a flightcase for the 4097 myself. Box of 9 mm waterproof plywood, aluminum-inforced edges, minimalist upholstery.
It turned out to weigh about 22 kilograms. add 11 kg for the horn and you are above your weight limit already, and with a considerably smaller horn than yours. This case survived a trip to china completely unharmed, so I dare say it does its job.
Professional true flight cases (at least the ones I´ve seen) tend to be of the same setup, be only a little prettier and cost about three times as much as mine (built with professional help).

The ones that may be lighter (fiber reinforced polymers) apply techniques that´ll call for even more cost and will make you VERY sad if messed up by flight personell.