On my CB 550 F there is also no gasket, but a drain pipe (top left, when you look down from a riding position). this will drain any rain water even while on the side stand. make sure its free by poking down an old clutch or throttle cable.
if you want to make a gasket, use something with closed pores, like neoprene, or foam rubber. ideally something strong enough to withstand modern fuel.
if you just want a cushion to make sure slamming/pressing on your tank lid wont scratch the tank, i'd suggest just protecting the corners, and not sealing off the whole lid. Otherwise you might block the tank ventilation.
