Lovely work James, and I concur about the wrapping of primaries, a friend once fully wrapped his exhaust that was going to run inside a glass fibre sill, "just in case" sadly just in case caused a pinhole of superheated gases bursting through a weld, onto the sill and complete flame out of car, poor guy. I "ceramiced" mine and it seems great, allegedly up to 30% less underbonnet heat. Only thing I did do once was very similar to your handiwork but made the alloy double thickness with a 5mm air gap, the top piece had very very little heat to it, or another idea is self adhesive turbo wrap (like silvered glassfibre mat) could be applied to bottom of your lovely work. Then again look what "belt and braces" caused my mate! Great solution.