First I tried a Hella slim line third brake light but I couldn't get myself to drill holes in my boot lid. So I fitted a magnetic pad under the light and this served as a temporary Hi Stop Light.
NOTE: when wiring in a third brake light you have to wire it up to the brake light switch with an inline fuse to protect it.
If you wire up to the taillight circuit at the rear of the car you will have a problem with your indicators, the wiring for the brakes goes through the indicator switch on its way to the bake lights. This is why you have to wire it at the brake switch.
Later I came up with the idea of fitting a red LED light strip that I bought on Ebay and mounted it to the thin stainless strip behind the back seat.
That way it could be seen with the roof up or down.
Perfectly visible with roof up.
Third Brake light using Red LEDs mounted and wired up, works well roof up or down.
If you want to have the cover on when the roof is folded down, the solution is to get one sewed into the cover or fitted to the stainless moulding.