Hello Stevemog,
I agree I have to do a suitcase mounted arrangement for when it is on the rack - in France it as an offence to block the light, so I must.
I like your light too. I did consider the location that you are using, but I was keen not to have much exposed wiring. I even looked into threading the wires through the rack structure, but couldn't see how it could be done.
Geoff's Shed at
http://www.servicestar.com.au/geoffsshed/High%20Level%20Brake%20Light.htmhas another version of a similar setup.
It's all six of one and half a dozen of the other and no two Morgans are exactly alike. I think it is the light unit that I found that encouraged me to get on with the job, the curved top and clean shape suited what I was looking for.
Happy motoring.