You should run an outerwear no matter what kind of filter you run. I have a Unifilter on my R and a prefilter from outerwears and I still managed to suck in a little sand after a day of riding. I don't run an airbox lid but it was a new filter that was freshly oiled. I just ordered an outerwears air box lid for a little extra protection and it still won't affect airflow or jetting.

 

I cant find outerwears for my filters, I know they make them I even have part numbers, but no one seems to have them. I guess I'll have to get a hold of a distributor and get some ordered. They are a must for a cotton type filter.