I use the rcm rear cover, and haven't used anything on the front for the year that I've been using my 1515 (mostly off road racing). No issues yet.
If I was going to use something I'd just cut out a bit of 1mm thick aluminum sheet that I have.
I might try it w/ a very thin screen to see what happens and to try it.
That is what i was going to recommend. I use nothing on mine but a rear cover and it is fine. I have been thinking of a screen but never got around to it.
I just use elec tape in the front, and a vinyl cap I snagged at work for the rear. You can actually get free samples of caps if you want and have some "business interest" ;)
The one I have is flanged, but I'm going to get a regular short cap in vinyl. It fits tight, and at worst a drop of CA holds it on. Poke holes for the wires and it will help retain the cap further and seal up tight. Nice as it keeps bearings clean
Electrical tape on the front and the RCM rear cover. I noticed that dust still got in the motor, so I just took both off and haven't had any problems. I think it runs a tad cooler too with no cover. I just blow it out after a few runs with air. This is running on a pretty dusty track.