Speaking for myself I've been disappointed in it too. The original foam was typically very porous and compressible. It was specifically engineered by the camera manufacturer to be a good brake, and good cushion, and a good vibration damper, all in one. All the other foam is denser. You have to tinker with foaming and re-foaming with pieces you have to slit, till it's just right; or as right as it can be. Too thick and it may jam the camera or be rough on the mirror hinges. Too narrow, and the mirror will over-travel its intended distance, being hard on the hinges once again. PITA.
Yes, I should have mentioned this. Felt trims aesily with a very sharp blade, I glue the trimmed surface. Contact cement - Weldwood or Pliobond or Barge - all work well.