You can make or have someone make a wooden, plastic or stone… “shim” (or wedge shaped filler) for it… the shim when placed in the holder at the bottom of the stone would make the top level, you may glue the shim to the hone if you like or just place the shim under it when you ready to hone… don’t waste the grits.

Back in the old days they would make a custom wooden box with the inside cut to match the unfinished side of the hone.