I have a Cervelo R3 and wanted to try one but none of them will fit on the rectangular shape of my frame so I made one myself (took about 10 minutes). I basically copied the N-Gear so it would fit on my frame. http://n-gear.com/

I took a 1" strip of metal and bent a "L" shape angle on it (about a 20 degree angle). I epoxied it to a small piece of rubber cut to the proper width so it would sit next to my inner chain ring. I connected it to my downtube with a couple of tie wraps. I haven't had a chain drop in over a year.