I saw that deal on the local mailing list, but I already have another one waiting at my FFL, and that will mean I'm two uppers behind the curve already.
I also have an pre-ban EA (secret code for "old"), cast, works fine. I always thought the discussions on cast vs forged vs billet were pretty silly. There's alot of other stuff that needs the attention first.
I always wanted to build a lower out of thick carboard just to show it could be done. It really just holds the parts in place, with the only major stress being on the area that holds the buffer extension. I bet you could get a few shots out of it, or quite a few if you used metal bushings to hold the fire control pins in place.