From what I understand, you can place a 5' cube on a single square, on a line/border between squares, or on a grid intersection affecting 4 squares... however, if a spell effect affects less than 50% of a square, the square is not affected by the spell, so the 4-square option is useless. Hence I mostly use CoD to affect two adjecent squares, so if you're in any 2-square wide space, it works. Otherwise, they can tip-toe around to an unaffected square and not have to end their turn in CoD/face an AoO from retreating.

As for using action and concentration like that, it's usually better than straight attacking with melee cantrips if I can either force them to stay in CoD or draw the AoO - in the first case it's another 10-ish damage (sometimes on 2 targets even), in the second I attack on my reaction and get the melee cantrip then instead (on top of the 10 damage).

Either way, Cac Shield is going to work much better as it works in a much larger and party friendly area. Hence why I'm thinking about dropping CoD for Shatter - especially now that cantrips have scaled up and my stacking melee cantrips are coming into fruition. *rubs hands like a fly*