i love building airboxes and filter setups from garbage laying around my basement. homemade is the way to go but mine definitely don't look the best

i'd try to think like the OEM does when making an airbox/snorkel/etc and follow as many of the standard rules of design as you can

does rain or road splash up easily enter the airbox? and when it does, does it get trapped, does it drain, or does it just pour right into the filter? you can always throw more plastic at it

is it easy to access the filter so it can be frequently cleaned/serviced/checked?

if it's a panel filter in a box is there enough room for heavy debris like bugs and moths to settle in the bottom until they can be removed, or are they just going to pile up and cling to the filter?

if the airbox is frame rather than engine mounted, is there enough flex in the tube between the intake and filter box so it doesn't break or yank the airbox out of place under maximum torque?

is it restrictive in any way, or does the airflow have to turn way too many corners to get where it's going? you can measure for heavy restrictions at full throttle with a vacuum gauge but to truly make sure the airbox isn't impeding airflow you'd want a manometer, personally i just make sure the air passage is always the same or bigger than the throttle body

does it SOUND GOOD? some engines have a symphony of beautiful intake noise and some require heavy muffling, and your airbox and its intake tube is pretty much your intake muffler, so your airbox choice could be the difference between a 'nice muffler' and a 'fart can' under your hood, crappy designs will whistle too

is the air 'fresh' or is it breathing air that's hotter than necessary? would it be hard to make the intake air fresh?

as far as k&n filters go, they don't let much dust through unless you don't service and oil them, i wouldn't worry too much about a tiny bit of dust, your engine makes carbon dust all on its own, a little pollen isn't going to make a big difference