Oil leak on primary cover (the clutch/left side)

I rescued a nice 1974 Commando and would now like to stop a small oil lead from the primary cover. I know some guys buy the belts but i'd like to avoid that expense. I have a new rubber gasket (the original tube type) but it's not working, still has a small leak. Any ideas on how to stop it from leaking?

leaks can be from the front mating to cases, the clutch felt 'seal' and the o-ring. Many find they have to put a sillycon layer in the groove to get the rubber band to stand proud enough to clamp a seal.