shouldn't be too hard to build a rectangular one. here's some links to parts you might want for it:
vinyl for outside of case
hinges
latches
handle

then you should be able to build the rest out of plywood with a 2x4 or something for the neck supports. then go to a fabric shop and buy something soft, protetctive but not damaging to your finish, for the interior of the case. and you should be good to go. I don't know how you'd make an accessory compartment, so you'll have to get creative.