I'm all for the extra brake light
on the drophead mine has been installed under the spoiler yep stuck a spoiler on it too [:D]
On the fixed head i'm sure you used to able to buy these from rimmers or someone like that but its at high level in the middle of the windscreen
trawl through ebay and see whats available
on the idea of the spoiler the peugeot 405 with brake light apparently was an option. but as you have neither a window or spoiler i would imagine a small LED set fixed to the middle underside of the boot is an idea. or if you are in for a bit of mutulation of the boot i'm sure with a little bit of bravardo you could actually fix one in the leading edge of the boot a bit like the MX5 cut and fill i'm afraid though.
How adventerous are you feeling Henk eh! go on make my day [:D]
[8 whole cylinders worth of punch to ram the world through the windshield and out the rear view mirror. Car & Driver]
1981 Grinnall TR7 v8
1981 Black FHC
1979 & 1980 Black Premium FHC(laid to rest) Sorry
John