Okay, now I know what the design is, and it's not what I had thought. It's more of a Colt Ace lockup but doesn't have the channel cut into the slide for the ejector to run in...which would run from the front, pinned to the barrel lugs. That slot I see which runs into the recess for the case rim is probably put there for the ejector. I think you're right too, about welding a piece on that will run in that slot and contact the spent case as the slide retracts and pulls the case into it. That's probably the way Bob Day designed it and you're missing one part...the ejector.
It's really interesting, I've never seen a conversion designed this way but it would definitely work. He'd probably have a little trouble marketing them in that configuration though, because it would require an exchange of the ejector every time the 1911 was converted. I think the Ace, Marvel, Advantage Arms, Kimber, is a better system.
I remember the Day Unit now, I've only seen one and that was at least ten or 15 years ago. I remember that little 'tab' on the front of the slide that slipped onto the frame's dust cover. I thought it was a bit Mickey Mouse...but it worked!
Sorry I couldn't help, but if you can find one of the units I mentioned, I suspect you could figure out how to mill your barrel lug for an ejector, and I'd still send you one, all hardened and ready to roll...and free! But for now, your welding skill is probably the best route.
Bob