Yes unfortunately Turbine offers very few native UI elements, many have to be implemented in Lua with the "low-level" control API.
I was in the same situation (for my plugin BuffIcons), and there is not really any implementation from the community that became standard either. There's almost as many variants as plugins using dropdowns and other "advanced" elements.
I have looked at ones Garan mentioned, aswell as Garan's own code. I did like the clean and simple code from Garan, but they don't aim for a Turbine-like look.
To make it short, I ended up using Pengoros' code (after asking for permission, of course, it is part of BuffBars for example) because it is also very nice and functional, and looks almost identical to the Turbine ones.
I only tweaked a few colors, and replaced the .tga files with the ressource IDs of the client-internal files. Though that doesn't really play well with custom skins I'm afraid, they end up neither unaffected nor fully skinned, but partially skinned :/
Couldn't get any information what's up with that.
For 100% Turbine-like look there are better ones, though I don't really see the point of lots of extra code and images just to recreate the original border that is just barely distinguishable from a solid grey one.