The REAL cause for losing air through the nose is arching the back of the tongue and directing the air flow to the nasal passages. Use the front of the tongue when you arch and the problem will solve itself.
Never use a nose clip.
Have you ever held your nose and blown gently to pop your ears?
Well playing a High C is NOT gentle blowing. This can be a dangerous thing to use. Nose plugs can prevent the pressures on each side of the eardrum from levelling off and cause you to blow out your eardrums.