Oh snap. I see that now. I didn’t catch that before. If it were me I would indeed try to apply a backer substrate of some type. You’ll just have to look around for something the proper thickness but things that come to mind are backer boars like you would use in a shower tile application or maybe hardy board? Or even attempt to just build it up with concrete but to do that you’d need to create some holes in the base that it sits in so that it has support and something to hold it better. Not sure about that though.