I primarily use the FELDHERR HALF-SIZE CASE 90 FOR BATTLETECH - 26 MECHS from Feldherr. I get mine through Fortess minis. They work great and let you fit two full companies of mechs with a couple of spares or have some cells for dice etc. Or if you have huge mechs like the king crab you could cut the divider out between two of the cells. The standard case in this size comes with two inserts, the first will hold 10 mechs of larger sizes. There is also a thin foam topper sheet to protect the minis from the case or impacts. With the 1- mech tray, Assaults like the Atlas, Banshee, Cyclops etc fit in here, sometimes a little snug but they fit. Bulkier mediums like the Thunderbolt or Orion fit better in this box as well or you can stuff 2 medium or light mechs in one cell comfortably.
The second foam insert holds 16 mechs. This tray has smaller compartments but will still fit most heavies without deforming too much. Thinner taller mechs like the Grasshopper, Catapult, Archer, Rifleman etc fit neatly in here. Warhammers, Marauders and squat wide mechs like the Dragon will fit but will deform the walls of the foam cell they are in.
You can also order separate foam compartments for this case if you want to mix and match your forces to tailor them towards storing assaults / bigger mechs easily or two of the 16 mech trays to store a larger number of smaller mechs in one case. The zippered cases are high quality and I wouldn't want to pack around anything bigger for my games.