I am using Flightline Interiors Firewall Insulation Fabric to insulate the interior of the firewall. I purchased 2 yards and will probably use a yard and a half to do the firewall and bottom of the tunnel

The material is very flexible with a metalized film on both sides.

It is about ½" thick.

Insulation of the FWF.

Right side, still need to make openings for the penetrations

Tunnel section with cutouts for the heater valves

The insulation is not permanently mounted. It will stay this way until after everything is installed in front and behind. I will do as Tim did and use 26B red Hi-Temp permatex to glue it in place. Just squirted some squiggly lines behind each panel and make an outline an inch or two from the edge of it and then stick the insulation panel in place. The red permatex should dry nice and quick too.

Tunnel looking down. Next step is to remove the fuel pump, filter and sender and install insulation on the bottom of the tunnel

I made paper templates prior to cutting the insulation.