We love our Clam, but it's not an add-a-room. Cheaper for sure, and probably more convenient to set up, but the unzipping and zipping every time you need to grab something from the trailer gets old fast.
ShadeMaker here in Chicago makes made-to-order screen and add-a-room conversions for their bag and roll up awnings (sold separately). The bag awnings simply slide into a rail, while the roll up awnings require a bit more work to install. The room panels attach with velcro. I'm sure there are other manufacturers but this is the one I've been eyeing for our oddball trailer.