I actually had the same worry and simply started modifying my gearbox shells... i say modify when i really mean mutillate but you can see the result
here
Worth noting though that I have one with the lower replacement and damn is it light - like insanely light.
And yes I've started selling the kits locally - I've been having a bit of trouble with some brands of upper though - the sizing is incredibly variable but some brands like G&P and G&G seem to be fairly relaibly sized lol.