I have bought many drink holders for a variety of boats over the years. I have usually been disappointed by either the ridiculous cost, the crappy design, or the lack of true functionality in general.
I needed two new ones after the two I bought died (One during installation, the other dashed a cold beer to the cockpit sole). I decided some lateral thought was needed and, surprisingly, considering lateral thinking is not my strong suit, I found the answer.
And, the answer is bloody brilliant! I bought some rail mounted holders designed primarily for Moms shoving baby carriages around the park.
A link follows. These things are inexpensive, well made and superbly functional. The lack of a fluid gymbal is of no concern and I suspect they will hang off the rail for years to come.