You could shim it on one side.
I have some stainless steel shim tape, it's great stuff. The kind I have is called "Matrix Super Shim Plus" and the website on it is
Home
Not sure if they are still selling it, but there are similar products to be found, just google "shim tape"
Also, Misumi makes some wider stuff in different thicknesses, never used it.
https://us.misumi-ec.com/vona2/detail/110300238820/
Is there any adjustment in the way the gantry attaches to the riser plates? Can you think of a way to make some?