Any recommendations on how to fix this chunk out the sidewall?
Step 1: Using a posidrive carefully remove binding from plate.
Step 2: Using posidrive carefully remove Piston Plate from ski.
Step 3: Place ski in dumpster.
Step 4: Mount plate and binding on different skis.
... or you could just try making a dam with duct tape and filling with marine epoxy, let it cure then sand it down. Use that as an outside edge from now on.