Start thinking 'outside the box.' Soooo many of my 'tack boxes' are really nothing more than rubbermaid containers....or buckets....or something I've found at the local discount store. If they hold what I need them to...they fit the bill! One of these 'carry-all' thingamabobs has a drawer in it that fits my hoof pick perfectly. I also carry a screwdriver (In case I have to fix a headstall), chicago screws, some leather ties, a few of those plastic tie down things, my leather punch...a scissors.
On the topside I carry the brushes, curry, flysprays, shampoo, etc. I don't necessarily throw that in the trailer every time I go out, but it does come in handy to just grab the carry-all, and all that stuff is right there! Or...I'll throw the stuff I do need into it.
I have nooooo idea what kind of 'tack space' is provided at the barns. You could pick up one of those huge plastic lidded boxes for your saddle/blankets, etc. (if it has wheels...all the better...if not...hey, it works!) I like to keep mothballs rolling around in the bottom. Keeps the mice out!