I ordered an arched mainspring housing from CDNN for the low low price of $15 plus shipping. I tried putting it in last night. One of the pin holes is either misaligned or simply too small and while I got the housing reassembled, I can't get it mounted on the gun.
I checked with the old housing. Sure enough the mainspring pin will drop right in. New housing? Nope, even with the guts taken out it won't go through. I should be able to fix this by opening that hole up with a needle file but I want to order a pin kit and a spring kit from Brownells first. I[m going to want them anyway and they're cheap.
The nice thing about the 1911 mainspring housing is that it really isn't a stressed part. When the gun is cocked, load from the mainspring is actually transmitted directly to frame through the housing retaining pin. That's why many of the manufacturers can get away with using polymer housings, especially on "lightweight" pistols. Aluminum also works just fine.
UPDATE: I opened up the pin hole with a some files I already have. Unfortunately it wasn't just too small, it was also misdrilled so the frame and mainspring housing holes won't align. More filework is necessary. Remind me not to buy a mainspring housing from John Masen (also known as the JTM Company) again.