Waiver of IP rights for Free Software

I need the company for whom I will be working to sign a waiver of their
rights in my Free Software development.  There used to be such a thing
on the FSF site, however the only reference to it I can find is a
message from RMS saying that he thought it was there but he can't find
it!

Can anyone point me to such a waiver, or send me a copy?

The points are:

  I will be working as contractor for a company which asserts its IP
  rights.

  They only really need those rights in their own area.
  
  It is not clear from the contract whether they actually try to claim
  ownership of what I do in my own time (in one place it implies that
  they don't, and in another it can be interpreted that they do).

  I want a waiver which they can sign saying that any work I do on Free
  Software projects is mine, not theirs.

(I'm not getting into the "you should only work for companies writing
Free Software" -- suffice it to say that (a) I would love to and (b) my
continued existence implies havng to eat and pay bills.)

Thanks,

Chris C