There are a few basic things you need in a mag box in a RUM class case, the base of the case needs guided up to the rails, you need the follower to be in the correct position so the step on the follower holds the case correctly under the rails, the case needs the correct access to the rails and the front needs to deliver the nose of the bullet clean up to the ramp. You can take a mag box that would otherwise work except for length and split it and epoxy it in place and you have an extended mag box, obviously you need to make sure it'll work so a little double sided tape to help hold it while getting it dialed work. Put a center feed follower on the bottom of the spring and run a rum follower on top and tune the spring position then clamp it down when it's not diving.
I've done a couple ultra lites that I ordered without mag cut out then I milled it out to make the box and glued two strips of thin metal on the front and back so the case would not bang into just stock foam under recoil, works with WSM size cases that you remover the top 1/4 inch of a normal box of anyways to get good access to the side rails.