I'm having a real problem hanging some kitchen wall units. The problem
is that the wall isn't straight. The first four units hang nicely and
butt up against each other front and back. But then the wall starts to
slope away slightly such that although the units butt up together at
the back, there is a 5mm gap at the front. I've been trying to pack
them at the back using a wedge made of successive layers of 4mm pine
strip, but I can't get it to look right. After 3 more units, the last
one is about 16mm away from the wall at one end.

Is there a better solution, other than rebuilding the wall?