What are peoples solutions to oil breather filters for non-stock air filters.

I know u can get small inline ones that just go on the oil breather, but can u just cable tie a bit of foam around it?

I don't wanna have to pay heaps for a little breather... although i do want it to have clean air.

Rumour has it some people just block em, but then i hear u can get collections of back pressure in the case, say from water vapour etc... which wouldn't be good i would imagine.

cheers guys,
chris.