Actually I don't think it does use foam ! The C330 uses threaded posts built into the body to adjust the height of the screen whereas the C33 uses "
shim washers on each screw for the focusing screen. If those were missing or put in wrong, you would have the problem you describe. " (see
here).
FWIW I found you can purchase a C33 repair manual from
here. Double check with them it is for a C33 (as per the image) because it appears incorrectly listed as the C330. It certainly isn't cheap, they list it for £26.95.
EDIT: Just found it cheaper on eBay
here. for US$19.20.