I would like a smaller can for hauling up and down the stairs for those big range days.  Have some big plastic ones but I got to the point where I inserted Styrofoam dividers so the mags would still stand up but reduce the weight of the can.   However, that means I am storing and transporting empty space.

Don't care for those cans with the generous foam spacers either, they are disposable items, I don't need them babied to the point where there is a bunch of wasted space.

30 cal width cans would be ideal since I have 3-4 different types of 556 ammo I like to keep loaded in mags ready to go.

Like he said, a .30 is what it is.   There are other solutions to mag storage that don't involve an old ammo can that work better and are lighter.    I like the basic stuff, a simple cordura bag that has 3 pouches holding 2 mags per pouch.

Anything like a repurposed ammo can will end up being what you have seen.

doesn't have to be steel, some of the commercial plastic ones are actually pretty rigid, but they are all about a half inch too short for the mags to be stored standing up.   I'd like a can vs. a nylon storage pouch because the cans are easier to label plus give a little more protection when loaded into a truck bed.

Look at the MTM Tactical Mag Can. Holds 15 mags, is plastic with a double latch. Mag stand on end in foam cutout. Have worked good for me. Amazon for less than $20.00.

It looks like you already got a pretty optimal option, but there's also this: https://www.monoprice.com/product?c_id=117&cp_id=11703&cs_id=1170305&p_id=10623&seq=1&format=2

Mags are 7.5" tall, just over 3" long. This case is big enough that you could fit twenty mags just about perfectly without using foam to separate them, a little less if you're going to make cutouts for them to fit into, just because you'll need enough foam in between to make it rigid. It's deep enough that you could fit the thin layer of foam on the bottom, too. IP67 rated, etc.