Please...read this with no ill will or snotty tone. Every time I've tried to reply, when I read it...it came off wrong.
People are 50/50 on the sturdiness of the box, this isn't designed for 'transport', but more so for long term storage.
As for the compartments not fitting your collection, that's honestly a failing of you not reading the dimensions of the compartments. All dimensions are listed in the product description, and the smaller trays weren't designed for mechs, they were designed for vehicles and infantry.
It's always best to see what the compartment trays are measured out as, so you know if XXX tray with XXX compartments will fit the miniatures you have. If any of the pre-built boxes or bags don't fit your needs...design your own by getting an empty bag and the selection of foam trays you need. That's honestly the only way you will get the exact fitment you require.