Cat-proofing my mattress?

I have a conventional box spring and mattress that I am looking to replace
because it is just too hard.

I'd like to know if anyone knows any ways that I can reliably cat-proof the
new mattress so that my two cats, both of which have their claws, don't
sharpen their claws on it the way they did with the current mattress. (I say
"did" because they don't scratch it a lot any more but I suspect that is
because they are out of places that they can scratch on the mattress that
aren't already shredded.)

While the mattress still works fine, it looks *awful*. They have shredded it
so badly that they can literally stick the top half of their bodies into the
holes they have made - I saw Bebop, the larger of the two cats, do it once!

I will not consider declawing them so please don't even suggest it. I do not
want to banish them from my bedroom or bed either. I'm looking for some way
to deter them from sharpening their claws on the wood frame of the mattress.
In case it makes a difference, they are both 10 years old, one is male and
one is female, and they are both neutered.

Does anyone have any reliable techniques for protecting the bed from their
claw-sharpening? If so, I'd love to hear from you!

--
Rhino