You may want to try nalgene (volara foam).  It comes in different depths,
1/8 inch or 1/4 inch,  it's flat with no "waves" as you might find in
microfoam or in 1/8 inch ethafoam.  It's white, although I have seen black.
Scientific companies generally carry this product in sheets but it is quite
expensive purchased this way.  I have bought it in rolls from Engineered
Foam.  Weston, Ontario Canada  (416) 746-7334.  It has been a number
of years since I last purchased any foam so I can't guarantee if this
contact is still good.  If you wanted to send me your snail-mail address I
would be happy to send you a sample.
Hope this is helpful.

Rosemary Holmes
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National Gallery of Canada
Ottawa, Ontario Canada
Art Storage Officer PSDMA