I have two range boxes: One is an old Winchester brand ammunition box that my brother found, and built for me. The second is a pickle box I found at an antique sale, and used to make my second box. Look at antique stores, antique sales, estate sales, etc. for an old wooden box.
There are NO SET measurements. If you visit a shooting range, like Friendship, you will find all kinds of shapes, and sizes of boxes being use- some nothing more than fishing tackle boxes that have been "Drafted" for this use. Nothing wrong with them. These boxes are either obviously some commercial product made for another use, or are "custom made" by the owner, or a friend, as indicated above.
This is NOT rocket science. Even a country lawyer like me can build one. I am sure you are quite capable, too. I FIRST decided what I wanted to carry in my box- not what others carried. I did have the benefit of owning the box my brother made up for me, but I don't fill its little trays like he does at all! Ifyou go back through the archives here under Accesories, you should find prior thread on this same topic, with pictures of boxes owned by other members. That should give you a couple of ideas of sizes.
Also- an afterthought-- You should take a look at the Cowboy Action Shooting ( CAS) sites on the Net. Those shooters also carry lots of gear in boxes- some made into wheeled carts-- when they travel to distant shoots. You can get lots of good ideas from the dealers that make boxes for that audience.
Formal Rimfire, and CF target shooters also have " Pistol boxes" that you can find on line, that will give you other ideas of what might be possible for your needs. I see similar boxes being use by the pistol shooters at Friendship- often handmade, because of the different requirements of MLing revolvers and pistols. A lot of championship level shooters shoot both BP and modern handguns to keep their skills up.