Easel back type material in sheets

[CB Art & Framing]

I'm looking for material I can cut to size for removable backings. Foam core is too soft.
Thought of PVC? or any other suggestions appreciated.
Needs to be acid neutral, but not conservation.
So probably not MDF or Masonite.

 

[Rick Granick]

Bainbridge's catalog lists an item called 4UC Double Thick Utility Mounting Board, but I can't find any specifications as to its actual thickness.
Rick

 

[GUMBY GCF]

coroplast?