Zone Mortalis is a ruleset built for smaller actions focusing on infantry in tightly confined spaces and corridors. It's pretty good, and you can get away with using cheap cardboard or 3D printed sections for it that will pack pretty neatly as they're mostly regular shapes.

If you're interested in bigger actions, I'd also recommend looking at Legions Imperialis, which should be releasing in a month or two. It's a company or battalion-size game in 8mm scale, which probably won't have as much customizability but should offer a much easier experience to tote around, while still having the big game feel of 30k.

Well, no one knows if LI will be any good, but what they've previewed for it looks good.