Peter,

I believe about ½” would be adequate for 90% of your baffling gaps.  However, during engine start and shut down you will still get an occasional contact of metal to glass.  This has spider cracked the gel coat on numerous cowls (over the years).  Perhaps ¾” would have been adequate.  However, around the top of number 1 & 2 cylinders I did not have that much clearance. ).  I built my baffles twice before shifting to a cooling plenum (duct) out of frustration with that contact damage.   In addition to better cooling ( big factor!) , the plenum allowed rounded edges, smoother contact surfaces, and overall less maintenance.

Larry

N360LH