Well, a minor necro to this post, mainly because I just made the 1x1x1 ML cube.
I made it as reasonably small as I thought I could, the centre cube is 12.5mm. As the ML switches are 12mm wide I could have gone a little smaller, but perhaps at the cost of some switch stability. The other concern I had was orientation of the switches, because I don't have any symmetrical (DSA/SA) ML caps (if they even exist), so orientation played a part.
The gaps between keys is probably as small as ML can go. When one is pressed it depresses further than the edges of the other keys, so blocks the adjacent ones being pressed fully. Rolls pretty well.
With coloured blanks, and with letters for a CTRL-ALT layout.
Basic block (Alu)
Block with switches