Besides Mesa and S&J, Ergo has one out. It retails for about $45.00 and I see that TacAccessories carries them. They aren't aluminum though. They are a high density polymer. They work well enough and I used to use them when I was doing M-14 stock mods. ATI also puts out some adapters for 870s.
I have to go with stjones though. The LEO is the best.