One would have thought the vent hole would be a drain at the lowest part back towards the seat area. I would seal it with oil resistant neoprene or cork. You could prep the whole inside and use thin film of motoroil, just like marine diesel engines of old to keep off the rust and moisture from the sea air from attacking metal parts. Did it all the time on my cars that had painted engine, oil pan and rocker cover tins...never had a problem with rust and kept the stock paint looking good. Since the fuel tank is vented already, and guages usually are not fully sealed to prevent fogging these days why not... JMO