securing battery in 456

I just replaced the battery in my 456 with an optima red top. The battery is located in the rear of the engine comparment on the passenger's side and requires removal of the radiator overflow. Anyway the optima is slightly smaller dimensioned than the interstate that was in there before and the edge of the battery that meets the shoe/clamp thing that ties it in place is very narrow. I can't get a good bite on the edge to where it (the battery) won't move side to side from a good shove. There's a plastic clamp on adaptor that comes with the optima to extend the edge at the bottom but with that on it goes too far and the bolt from the clamp down can't make it past the plastic. Perhaps I shouldn't worry about some side to side play or perhaps my testing with a shove is more lateral force than would normally be encountered but I'd frankly prefer to see it clamped forcefully in place. Anyone encounter this before? I suppose I could fabricate the plastic extender to fit?