I saw your post for a cheap cargo carrier. I still have photos of the roof rack I built for my Rambler you asked for a while back. The problem is, I don't know how to attach photos to this site, I'm not signed up for Photo Bucket, Drop Box or anything else. . .
Drop me an e-mail to firstname.lastname@example.org and I'll send you the photos. It holds my wife's bike, my Rambler and I'm only into it for about $50 worth of broken bike racks I found on Craigslist, plus some time and miscellaneous hardware.
If you decide to go with a hitch platform, you can add a rail to hold the 3rd wheel; left and rear on the platform with the right wheel on whatever extension you can manage. I'm new to trikes, but I know how to save money!