Gasket material

This car was modified to wheel chair accessible from the rear. In the conversion, in order to install the ramp and the wheel chair cavity, the original gas tank from factory was removed and it was installed a customized tank. I have problems of leaks of fumes and gas smells and I found that the gaskets for the fuel pump assembly and vent valve were totally deteriorated. Those are not commercial parts that can be found in any auto-parts store. I had to make those gaskets myself. In Grainder I bought a rubber sheet of neoprene of one quarter inch thick and made the gasket. Is that material okay? Or should I use any other type of material?