Napp
I went through essentially the same process you are enjoying right now. I must have tried six different carrying framuses including a Plano box designed just for shooting. I found that although everything was close, nothing was right. I finally wound up making my own as many shooters do.
My process was to closely analyze the things I took along shooting, not at the house but at the range. As I went through that process I picked up (at least mentally) everything in the range to make sure I needed it. This process resulted in a mental list of the things I wanted to put inside the box.
(I don't carry the pistols in the box. I am working on a different box for those.)
One of the things that was important to me was the Chronograph and tripod. This single item established the length of the box.
The depth of the box depended upon the height of a pound of powder. The width is just a convenient measurement that makes the proportions of the box look right and allows room for the tray alongside the powder.
I have not yet put handles on the box because I can't decide where they should be. (Ends or top). I am leaning toward two rather beefy handles one on each end.
I did not want a box on wheels because if it has to be on wheels it is likely too big to pick up and put in the truck. And probably too tall to stand up on its own in a moving pickup. Mine is light enough to carry and low enough not to tip over.
The food tubs hold balls and lube. The pill containers have different size caps.
The other end of the box has the loading press.