I'd be cautious of what a shotgun sizer could do for you. I think your chamber is tapered, and it doesn't sound like you have even been able to get to the base which is maybe all the commercial sizer would do.
If you want to go low budget, maybe you could drill a few holes in scrap steel plate, based using some indicating die and measurements of where the case is hanging up in the chamber. I'd bet you could create a series of mild steps to get the brass to chamber, then fire form and trim before loading. Who knows, but I would suspect that the web of a brass case may be hard on conventional shotgun sizing equipment. Only thoughts.
