The plenum drain is their in case the carbs overflow and fill it with raw gas. For proper function it needs to allow liquids to pass, but not let significant air bypass the filter system. Just about any air filter foam, as used in lawn equipment, cut to size and stuffed in the end will do the job nicely. Note, the original aose was molded and reduced in diameter at the exit, to limit air volume at inlet. "Stacking or nesting hose sections at the end will do the same, and allow a means to keep from sucking the foam into the engine.
Cheers,