At 67, I wouldn't bother trying to add more, but you could experiment with partially blocking a vent hole, if it drops any lower.
 
I don't think sponge type matters. Just a clean one, I just rinse mine in super hot water to sterilize.
	
		
			
		
		
	
				
			I don't think sponge type matters. Just a clean one, I just rinse mine in super hot water to sterilize.
	
 , gig.