Open air area in stock airbox?

I'm thinking about designing a custom airbox for my KZ750H3. What I'm trying to figure out is the total area of access to air in the stock box. Under the filter cap is a plastic stem with a number of small holes in the sides. Other then the crankcase breather hose, are these little holes the total access to air that the filter has? I assume that the stem goes all the way through the filter, blocking off any other access to air. Is that right? Thanks!

The air box cap on the 750H3 is the same part as what's used on the 650s.
On mine, the stem part goes about 1/2 way through the filter, so the bottom 1/2 of the filter is open to breath.
Some have added holes to the stem, others have cut the stem almost completely off.

After doing a lot of measuring and calculations, it appears that a custom box with an opening of 4-3/8" by 2", or if different space is needed, an opening of 3" by 2-3/4", using the same basic filtering materials, would provide the same amount of air access as the stock box. Then the opening could be made smaller or larger depending on the engine's reaction.

Anyone see a reason to disagree?

Your bike has CV carbs. correct? ... it seems worth mentioning.

Yes, the usual CV34's, stock jetting (I think) and stock exhaust.