Your idea is good, I think the clear shade on my picture is due to the bubble itself one of the magnifier, and on this area, frosted and not frosted are identical, I admit I was thinking about something like this when I saw the lighter shade on the area, but not sure about it.
Thickness could be the solution.
If I remember correctly frosted have two steps window, one step on each end, making a 2" large window to allow the red plastic parts underneath, and then a second window, same size as the bubble rank alone.
It would be interesting to know the frosted bubble thicnkess with no trimming at all once the ends are cut, I think it may be a bit thinner than the most common bubble, meaning no need to sand it down for a obi saber, just tapering and ends cutting.
What makes me think obi bubble can be a frosted is more the vader anh lightsaber, it has a frosted bubble type exactra board, but without bubble, so the bubble can be somewhere else, obi saber?