Make your bullet then send it to RCBS and ask them to make you a mode.
I have a mold for making 375 bullets. I cut a piece of 3/8 copper tubing to fit the mold, the pour in the lead. It makes a great jacked bullet right at 270 grains. Accurate in my 375 H&H.
You can do the same thing with 5/16s tubing.