Since the new aftermarket insulator I installed was hard plastic (or similar material), I went with a gasket on both the top and bottom surfaces.
When making your gasket, make sure to add a small vacuum hole in the gasket to match the vacuum hole in the bottom of the carb. Roch B's utilize a vacuum actuated power piston to add more fuel to the engine under load. It gets this vacuum from the small hole on the bottom of the carb flange where it bolts to the manifold. You'll notice the OEM gaskets and insulator has this hole and a small channel cut in them for this purpose, just match it.
Good luck
Dave
