The VooDoo 3Gun Bag is the jam. Built super-well and will last for quite a while unless you do something stupid to it.
Long enough for the 12rd tubes. Plenty of internal dividers, magazine pouches, external pockets, external loops for stashing stuff, velcro for whatever you may want to use it for, etc.
I had to order mine off of "Major Surplus" instead of the actual VooDoo website.
They aren't cheap, but they are absolutely worth it.