Hi Silvie, I was just raeading your post and remembered that Michael Covington in his "Astrophotography for the Amateur" mentions this specific problem (foam rot). He says that he fixed it himself, but then goes on to say on page 159
[quote]
Complete foam replacement is a tedious job best left to a professional, but you can do piecemeal repairs yourself. I made a nice mirror pad for the camera in Fig. 9.4 out of some brown foam furiture padding from a hardware store. Since its not in the image-foming light path, the brown color does not matter.
[end quote]
Unfortunately he doesn't go into details
If you can get this book at the library you maybe able to pick more details out of it.
ps Mike how does one turn these things into proper quote formatting as I've seen in ohter postings. (computer language illiterate)