Well, there is no government certification for ultralights. It's not required by the regs, nor is any provision actually made for certifying them.
I'm rather fond of the
SkyPup, myself. Usually I hear good things about it, even though it's an older design.
Another one that's spoken of often here is the
AffordaPlane. I've heard mixed results about dealing with the proprietor, though - in general, the gist seems to be to order the plans electronically and not on paper. Some people who have ordered paper plans have reported delays in receiving them. I ordered a set of the electronic plans out of curiosity about the design (at $25, it was worth it just for that purpose) and they were promptly delivered within 24h.
Of the two, I'd probably build the SkyPup myself, but that's just because I'm a little more comfortable with working wood than metal. Your mileage may vary on that.
Neither of these is in the pod-and-boom pusher configuration that you'd originally mentioned, but I don't happen to know of any ultralights in that configuration that are available as plans.
In either case, you mentioned needing "flexible" plans. As with flight training, there really isn't any such animal. What's done on the plans was generally done for a really good reason, even if that isn't immediately obvious.
This also applies to the materials chosen. Scrounging is fine, so long as you're scrounging for
exactly what's called out on the drawings. Don't substitute. Your life is on the line, literally.