It shouldn't be that hard to modify the factory air filter but the trick is getting the filter box height correct so all the mount holes remain aligned when it's bolted back down. I'd get a factory box from a JY and butcher it so it can be jigged until it seals to the Weber and isn't being pulled out of shape. A scrap piece of alu sheeting for the adapter and a squirt of RTV silicon will seal it up airtight.