Let me make this suggestion. instead of doing a signature block, why not use muslin and have the guests write on it as the back. they can write more, say more and write as big as they want. I did this for my mother's 90th birthday and then did a regular patchwork for the front.
It made a good lap quilt and or wallhanging and she didn't have to look at it and see all the deaths, divorces etc etc breakups that occurred in the family in later years.
and best of all you don't have to rush to get it finished.