Spurred on by Dave W and others added to this thread I have just completed fitting my version of the Third Brake Light for use with a Suitcase.

The unit in place and lit.
The strap is made on a sewing machine using a 'reused' Black Webbing Laptop Case Strap, doubled around lengthwise with an alloy plate drilled & inserted to hold the fixing bolts.

The Parts Partly assembled.

The finished components including a hook to retrieve the cable from the wheel well and a bag to keep it in when not used.
The cable uses an inline connecting plug and socket with the car end held when unused with velcro (someone elses idea - thanks)

The Strap in place. The cable is held between the Frank 4x4 mark 1 Boat Bag and Suitcase cover.
I have sourced a capped socket and plug which will be better but requires drilling a hole in the rear bulkhead behind the seats (one for later)

A closer view of the lamp. The flimsy cables are covered with a rubber sleeve and wrapped for durability. Not very pretty but effective.
The lamp itself has a swivel feature which is why I bought this model. This allows me to place the strap over the top of the suitcase and set the lamp angle to point rearwards.