Just finished some slightly more travel-friendly gameable pieces than the Makita case, in that I can confirm that there is space for some socks around the box if crammed into a suitcase. The same charity shop had a box which, it turns out, is just the right size to snugly contain some A**zon-ordered trays.
The trays have a bigger playing area than the double-sided picture frame board featured in this thread's first post, and this time round I put more care into not putting too thick a layer of texture so that magnets work a bit better.
The turf tray didn't turn as well as I'd intended, but I'm pretty happy with the paved one. Good to put some scrap Magic The Gathering cards to good use.
Space to fit at least two more trays in there, or some clever way of storing terrain less than 3cm tall.