2) Make up a full set of shims (inside and outside) before fitting
any flush drawer. Remember to give more room to the bottom to
overcome the optical illusion that makes it look narrow.

That one sailed by at least six feet over my head ... maybe a
vocabulary thing though ... to me a shim is a thin spacer piece to pack
out things that don't quite fit. I'm sure you must be meaning something
else !!

:-)