Unfortunately, there's just not a whole lot out there for milled receivers.
It's not really practical to modify the milled receiver to accept the folder pictured above (it would actually be easier and cheaper to rebuild the entire rifle on a $500+ Firing Line milled receiver designed to accept it!
), though K-Var might sell that stock attached to a "folding block" that mounts to the rear of the receiver. It looks like ass and I don't think they make one for a milled receiver anyway.
You could also go with ACE, but they're really big and bulky and require you to cut the tangs off your receiver.
Otherwise, you're limited to just various plastic and wood solid stocks.
ETA: Well, one more thing. I beleive you could mount an underfolding stock. Cut the tangs off and have the receiver drilled for it. I don't think the cuts are very complex, people have even done it at home, albiet on stamped receivers. It's still something you'll probably want to send out though. Makes a scope rail a no-go though.
ACE is probably your most practical option.