Air filter custom airbox

Good Afternnon, 

I wonderind to build an airbox using round filter ( K&n) fixed on side of engine cowl taking fresh air from naca scoop.

Any one did it? Any suggestion for filter size? 

I think one like this is enought 

https://www.knfiltri.com/e-2566-special-order-repl-fltr

Otherwise take an alredy done airbox 

https://www.repartocorsestore.it/FILTRO-ARIA-SPORTIVO-BMC-CDA--ACCDA70-130--AIRBOX-ASPIRAZIONE-DIRETTA-CARBONIO--CONSIGLIATO-PER-CILINDRATE-SINO-A-1600cc--SPEDIZIONE-GRATUITA_p_221219.html

On this case alternate air will be not so easy to do

Grazie

MArco