Tyvek is not a viable candidate.  It decomposes badly under extended
ultraviolet exposure (sunshine).

Mylar may have a problem with long-term decomposition (20 to 30 year
period) depending on its molecular structure.  I can't comment further;
I just remember problems with some brands of professional audio tape
breaking down from misformulation in the 1970's, 80's.

Jim Cook
Bolton, Mass.

 [ 1/4" acetate recording tape of the 1950s simply snapped when
 [ overstressed, and it was easy to repair the break.  I cried upon
 [ my first accident with the new 1/4" Mylar tape: it stretched instead
 [ of breaking!  ;-(  -- Robbie