The extra factor is still being able to bottom-feed. The fast food cup holder would probably get soggy if it was soaking in hydro nutrients half the time. The rocks would work, though at that point it's like adding extra medium after the fact (which isn't necessarily a bad thing, but once roots grow into it the setup changes).
For now I have the plastic sleeve cut from the middle third or so of a 1L bottle reinforcing/supporting the solo cup, with openings in the bottom to allow water flow. I'll see if I can figure out a better waterproof stand.
And, yeah, it's a bit of a silly problem.