If you get concerned about exposing the filter directly to the elements, why not try a rain sock ?
This sort of thing :
https://www.ebay.co.uk/itm/Pre-Filter-Rain-Sock-for-Stealth-Air-Cleaner-Kit-S-S-Cycle-170-0193/184217887475?epid=202987013&hash=item2ae43de2f3:g:SKgAAOSweZ9ec9Ns
I got mine from Matts Machine Shop , cheaper as well from what I remember.
I have one fitted for many years with my Morgan Stage 2 filter where the filter element is completely exposed ( and been caught in torrential rain a number of times ) with no issues.
I use a rain sock on my S&S Stealth filter primarily to stop the dust which is my biggest issue on the dirt tracks up here, it traps an amazing amount of debris which is easier to clean off than cleaning the filter.
