I'll offer a different perspective...
Not knowing the details of what you're being asked to sign/seal, I'd like to know the risks before saying "hell no".
Bottom line: If they asked you to sign while you were still under contract, would you have done so? If yes, then think about it now assuming: 1) it is legal, 2) there is little risk, and 3) you wouldn't need to spend significant time reviewing the final product. Of course, if they were willing to fairly compensate you if it did require significant time, then all the better! But then I'd wonder if it's worth the hassle of addressing the method of payment, second-order affects (like liability!) and any tax implications.
Something about burning bridges, cutting of your nose to spite your face, or something similar.
To be clear, in no way are you OBLIGATED (which you particularly highlighted as the question) to do anything for anyone... *EVER*. Had they asked you to do so while you were still employed, you weren't then and you aren't now. But they could have tried to terminate your contract if you failed to perform.