In the ATF's eyes, a 20% incomplete ANYTHING is a hunk of metal. I have read someone's opine elsewhere online that there is a "no trigger" rule, as in it must be impossible to install a trigger without further completion.
http://www.tacticalmachining.com/80-...-receiver.html
This is a company that sells them, one of many I'm sure, and it seems that the price is affordable. I only own old wood and steel rifles so I have no idea the cost, but this should help put you on the path.
Godspeed, brother!