In the UK we would be more likely to use Plastazote polythene foam to create a pad. It is blown with nitrogen to even the cut bubbles are inert and is available in different grades of stiffness, so you could choose one that’s right for this situation. Polyester or polypropylene felt are sometimes used and can be very thick. Acrylic felt is also available, but I would be cautious about this, as I’m not sure what acrylates are in it and how it withstands pressure and possible changes in moisture.
Whatever you use, it would be good to have it pressure tested between two plates, ideally of steel and iron, to see how it behaves. Many felts are given coating (fireproofing etc) which could affect the objects.
You could ask Sunderland felt for help http://www.sutherlandfelt.com/ They make felt up to 2 1/2 inches thick !
Hope this helps.