[TR] radiator duct material

[terryrs at comcast.net]

Five years ago, I installed a cardboard radiator duct in my TR3A.  Did it as right as I could, spray painting it multiple coats with undercarriage spray.

Disintegrated last year.

Just bought a new one, but plan to use it as a template to build a longer lasting one.  Any thoughts as to the proper material?  Some kind of readily available mylar, something like that????

Terry Smith, '59 TR3A
New Hampshire