Nah, they'll be fine open. unless you've got some kind of massive oil leak at the back of the head or cam cover that would let it druel down into those holes and wet the clutch down. (I've had open ones all summer since I converted to aftermarket EFI and removed the sensors. Been fine, even in downpouring rain)
Still, if you feel compelled anyway, a bit of sheet metal and some short bolts will cover them up nicely.