I doubt anyone has.
You might be able to buy all the parts separate and build your own for a bit less, but there would be no reason IMHO to try and fabricate any of the parts. Most have specific function or are simply cheap, common plumbing pieces anyway. The filter bodies and cartridges, the membrane, the flow reducer etc are essential and can't be DIY'ed around, the rest is cheap elbows, on/off valves and tubing, it's really a pretty simple contraption made mostly of essential manufactured parts. The housings are designed to hold a lot of pressure, to me it would seem silly to try and fabricate them the way we do for DIY canister filters. It would make sense if you could get these parts at discount or as free hand offs, but otherwise it probably makes more sense to buy one complete. The $99 ebay units are fully effective for planted tanks and the quality (and sometimes brand) of all these pieces are basically the same no matter who assembles them.
At the moment I'm refurbishing an old trashed Kent that was given to me, by buying the missing pieces from different places as cheap as I can find them (or free). That's as close as I am to "DIYing" an RO unit.