The professionals would leave a gap, put in an offcut of board on top of the plinth and tell you it's a shelf rack.
I'd suggest taking the side off the 150mm unit, and extending it sideways with board and a piece of plinth. If the units are the type where you buy the carcass and the doors seperately, buy a wider unit and cut it down to fit with a 150mm door and a filler made from plinth.