I picked up a nice soft case from Dave's Baggage Train at Historicon. It has 8 trays in it (different heights and they stack on top of each other), I am putting steel sheet on the bottom of the trays and my miniature bases have magnets in them also. I am doing the hole in the base to put a rare earth magnet. I have 2 15mm DBA armies but I do a lot of 3mm and 6mm also with a little 28mm thrown in.
Once I fill out the case, I will get more trays and if I do any serious travel with it, I can get a hard case that the trays fit into.
Like Tony, all of my elements have magnetic bases. When I travel to tournaments by plane, domestic or international, I use Plano tackle boxes with self adhesive steel paper lining the bottom of each box. They are then placed in a soft bag, together with rules, cheat sheets and measuring devices, which I can use as carry-on luggage that will fit into the overhead lockers. When traveling by car I use a single tier metal toolbox that will easily accommodate 4 armies.
Dienekes, remarked with a laugh, 'Good. Then we'll have our battle in the shade.' - Herodotus, The Histories, 7.226
From The Container Store. The width of the compartments is about 42 mm. You could put a magnetic tape on the bottom to further help hold the armies. The only issue is that if you use really thick bases then some of the spears or elephants may not quite fit the 2 and a half inch height. Might have to cut down some pikes. Because the width is so close to 40mm I find that I don't have to worry too much about the figures jostling about.
I use plastic tool boxes to accomodate the little fellows,varying the size depending on the number of elements, elephants and skull towers (Timurids) I have. Some armies I still transport in old box files, notably my Normans as I can carry their camp and Lombard Allies/Opponents in the same box.