Good call regarding the rear battery filter.
However, I know of one guy out in the desert who washes, drys, and replaces his stock HV battery filter.

After 5 or so washings, he says it is still holding up fine.
It is made out of some fabric, and is not paper like the engine filter.
You could try that if you wish.

Talking with Ford, it's not a mission critical filter anyway.
They say it's mostly there to keep dried leaves and insects out of the air duct.