Hi Jean,
I’m terribly sorry for your loss. If I were to attempt this project, I would use stone. You are right in that wood will warp. Metal isn’t a good choice either because it will act like a heat sink and get quite hot in the sun. Depending on the resin you intend to use, it may even warp or melt when in contact with the metal. Stone shouldn’t get as hot as metal but will retain its shape. Perhaps there is a granite shop close to you that has some scrap pieces you can work with?
You should also be aware that in the bright sunlight, the resin will yellow. How quickly is difficult to predict however. Just know that at some point you may need to redo the project.