At one time I was going to build a DR using the original Holland & Holland .375 2 1/2" Flanged Nitro Express cartridge. There are some of those brass cases floating around as I sold all that I had as well as the chamber reamer. This is a wonderful old cartridge and is still well regarded. The rim diameter is around .523" and I think the base is considerable smaller around .456" or so. That rim and base size might work for you.