If you plug the mounting hole in the top of the air cleaner box you might not even need the centre bolt. It's primary purpose is to clamp the whole air cleaner assembly down to the carb but if the base of it is secured rock solid, you won't need to factor it in. Hmm, the Weber sits higher than the stock carb with the adapter under it - I didn't know that...

This will work better than the open element filter and maybe with some heat shielding in it (or even around the main intake duct as it's hanging over the exhaust manifold), it might aid intake temperatures further