We do not have a spacer available. By keeping the track flush with the face of the rail, it eats up a lot of real estate in the bed.I like your solution but I went factory since a lot of aftermarket accessories seem to want the "face" of the rail to be flush with the lip of the bed. If yours has one it would be nice if you could develop a spacer that would accommodate any gap since your tie downs seem to be a better solution than the factory.
I went factory since I was installing BillieBars. (copied the image from their website).