Signal Vehicle Products (SVP) has the thinnest LED lighthead I have ever seen. I have 4 of them installed in my grille and they are SUPER thin and VERY BRIGHT. Mine are in RED. They are about 1/2 as then as the Tomar Rec 13 and have a much bigger footprint than the Rec13/14's. The lightheads I have are about 2 inches by 4 inches and about 1/2 of an inch thick at the very most. VERY thin! The model is DLXT "Thinline" Series. The ones that I have to not have the bezel as shown on the website. They offer "L" brackets also or they can be surface mounted easily. Self contained with 11 flash patterns.
I hate to play devil's advocate here...but why are you so concerned with using the smallest light you can find? If you're legitimately concerned about visibility and the lighthead standing out against the background you'll want something with some surface area to it.
Not trying to dash your hopes and dreams, just trying to give you something to think about.