Having recently joined the 4 eyes society, I tried running with my contacts behind my goggles awhile back. That was NOT a good experience.
Searching the web,there are a number of places offering varied forms of inserts for goggles.
I called several, wasnt very impressed. Then I stumbled upon Drew at 888rxgoggles.com. Within 5 minutes, you know he's just got his act together. An avid paintballer and mx'r, he's got a great system, and knows goggles as well.
Spent probably 30m on the phone, explaining goggles, etc. He's also the only person I spoke with that not only suggested, but pretty much insisted on getting tearoff's. His logic was simple, tearoffs are cheaper than a goggle lens. Yesterdays ride proved that, as a branch wacked my goggles, and the tear off got scratched, but not my goggle lens.
Drews goggle insert, while looking kinda wacky, is easy to get in and out without tools.
The goggle choice he sent me was outstanding, and after yesterdays super dusty ride, only a small amount of dust was inside on the goggles, and I was getting dusted big time.
I love them.