I've been making my own blocks for a long time. I use whatever piece of attractive wood I can find, oak, cedar , juniper what have you. Usually cut it 3" wide, 1 1/2" thick and what ever length suits you. If you have access to a drill press buy a Forstner type drill bit(s). They cut clean flat bottomed holes without any edge tearing. I've made some blocks drilled 1/2", some 9/16/ some 5/8 and it sounds like you might need 11/16.
Last night I made two blocks tapered from 4" high at the back to 1 1/2'high at the front. All holes drilled the same depth into the wood from the top. The cartridges sit in stepped rows and it is very easy to remove any individual cartridge even with my clumsy paws. Mind you I've got time to spend making little doodads. besto
Take a case to your local gun store and try it in some shotgun trays. I couldn't find a tray to fit the 7.62x54R so, I got a 50 shell, 20ga model. Works great!