Not going to lie, I did this for 20$, and it works.
The UTG, barrel mounted bipod, they call it a "Universal Barrel Mount" now, will fit on the M14.
Or at least, it will if it has the same dimensions of my Echo. I had one lying around, and figured, what the heck, I will give this a try, and it actually fits quite nicely onto, what would be on a real M14,the Gas Cylinder, and between the barrel.
Looks great, works well, did it for under 20$
IMO, the Harris style of bipod would look more realistic, and the bipod adapter would put it in a better spot on the gun, where you could get a realistic looking Bipod, with a great range of movement, but, if your stripped for cash, the UTG Under barrel works just fine.
http://www.airsoftgi.com/product_info.php?products_id=855