Hi there Jean,
I'm so sorry to hear of your loss with your beloved dog.
There is a poem called "The Rainbow Bridge" that is very popular at a time like this:
Also on that site, there is a page regarding pet loss.
I read a few poems on that site and although they got me crying, I don't know if any of them would be good for your memorial.
Here's another site that might be helpful too:
Sending my best during this hard time,