I think the only feasible way is to make it vacuum formed. A competent carpenter can make the buck, or you could mill one with a cnc machine from a 3d model.
I would use internals from ricochet apache or whatever the single 9 volt version is.
Those a already used by tippmann and kingman IIRC, so you could maybe get actual licence for them, and buy internals as a bulk buy.
Also they have a really simple operation, that would not need too much precision from the shells, as the vacuum formed pieces are not really precise.