Typically you make cases with ball corners on the top and "dished ball" corners on the bottom (lids) - that way cases of the same width stack. They don't normally mechanically fasten but you can get lashing straps and attach them together, or as an alternative you can use tie downs. Lashing straps are a good thing to have, as they are basically tie downs without hardware used for bundling things together. This is probably a good idea if you truck pack.
I'll have to get some pictures of my trailer - I think I have a really good setup. It's not huge, 7x16 - but I do fit a lot of crap in it. I have E-track on the walls at 1' high, 3' high, and 5' high. I can put wedges on top of a stack of 2 subs and strap them in with the E-Track.
I've only been doing this a couple years - but I've learned in that short time that packing your gear is probably THE most important thing. I added casters to my subs, all of my lights are in rolling cases, there's only a few items that don't wheel around. It makes a gigantic difference.