For a LED stop/turn/tail light, consider the 7" bus units. Grote, Truck-Lite and Peterson (and probably others) should all make comparable units, designed to mount on flat surfaces. Who could miss them? An example of what I'm thinking of is at the bottom of page 78 of this catalogue.
http://www.grote.com/marketing/Liter...Turn-Lamps.pdf
To be even more conspicuous, mount them on a removable board covered in reflective sheeting.
http://multimedia.3m.com/mws/mediawe...SevTSeSSSSSS--