Rather than spending the ridiculous $79 for the GoPro aviation filter made by NFlightCam, I decided to make my own. I ordered a .6 neutral density filter sheet from Amazon for $7 and simply cut a round piece that fits inside the waterproof case in front of the lens. If you want to completely eliminate the prop (blur the prop) and see what the naked eye sees, simply cut two pieces and place in front of lens, or order a 1.5 ND filter. I also added a the filter inside my iPhone case in front of the lens. Works great there too!
http://www.amazon.com/gp/product/B000B73OL4/ref=oh_details_o00_s00_i00?ie=UTF8&psc=1
http://www.amazon.com/Lee-Neutral-Density-Lighting-Filter/dp/B000B73OM8/ref=sr_1_1?ie=UTF8&qid=1371733635&sr=8-1&keywords=20x24+1.2+neutral+density+filter