Greg,
The technique you describe works rather well when 'zero' thickness insulation is needed.  On my old cowl I had the same problem, especially after installing softer mounts and reshimming the engine.  I used aluminum sheet bonded around the perimeter with BID.  Aluminum is much easier to form.  It protected the area well  for the six year I flew that cowl.
Chris Zavatson
N91CZ
360 std