I tried the national match rest. It worked ok, but I shoot more accurately without it, even with big bores. So I gave it away about 2 months ago. If I had known you wanted one, I would have given it to you.

Frankly speaking, you don't need to do much work to develop a load for an elephant gun assuming that you are starting with published data. Just about any load will shoot more accurately than you can hold offhand.