Not wanting to be too pedantic, but if yours is an ‘07 it can’t be a Roadster; they first appeared in 2010. Yours will be either a Classic (2 tone paint, foot boards instead of pegs, pull back handle bar, dual seat) or a Standard (single colour, pegs, flatter bars, twin seats). Of course all of those things could have been changed out.
Anyway, there are 2 styles of backrest pad to fit the OEM sissy bar - see pics. The larger one is generally known as the Classic (and there was two types of that as well, including a comfort option). Both sizes fit on the low sissy bar.