I'd recommend adhering your tiles with commercial 'thinset' to concrete stepping stones purchased from a building supply store. I've used 15" x 3" round concrete stepping stones and then made mosaic tiles to be added to the top. My handmade tile pieces are about 3/8" thick. Easy to dry and fire; and when coupled with the concrete base, they're very strong. Not sure how well it would work if you were trying to adhere one large tile. Using a mosaic approach allows you to adhere the smaller tiles, making sure there are no voids. Then grout around the mosaic. Using the mosaic approach might reduce the potential for one very large ceramic tile to crack. My stepping stones have been outside for 2 winters in -5F degree temps with no problems.
Perhaps make 2 of them, install and see how they do over the course of a year. Adjust your technique if needed and then do the rest.
Good luck! Nairda