Got a second hand storage heater from a friend

Had to remove the firebricks to transport it, as it weighs a bloody ton.

Anyway, there are insulation panels front and back that go between the fire
bricks and the metal case.

Trouble is, the material has split and the cork(?) insulation (better not be
asbestos) has spilled out, so need to replace.

Having difficulty tracking down replacements and if I've read the
description on the Dimplex website correctly, replacement insulation panels
seem extremely expensive (unless I read the wrong part).

So, any idea how to improvise a suitable replacement sheet of insulation?
Needs to be fairly thin (about 1/4")

Thanks in advance

--
Best Wishes
Simon Taylor