Hmm. I'd always wondered if you couldn't load a bunker with units, then move the bunker, and center locations on the units that were in it (which due to game glitch are able to shoot from the original spot the bunker was in when they loaded into it, so they might still be treated like they're still there by the triggers). Dunno if that'd work for a grid, though (or for your purposes for that matter).
Yeah, edge-location grids are probably your best bet. Just suck it up and learn 'em.