In 2003 I fitted a simple, but effective lock to my MGA boot. The basic idea was explained to me by my MGA specialist supplier - Bob West. I just altered the detail very slightly.
All that is required is an MGB-style glove box lock and a small U-clamp. I did not even have to go to Moss for the lock; my local ironmonger keeps them in stock.
The only destructive work required on the car is the drilling of a suitable diameter hole above the boot latch pull to accept the lock (see photo).
The lock lever is then notched to fit round the operating rod and the U-clamp (6mm inside diameter) bolted in place (see photo). The exact positioning of the clamp is trial and error. It is important that a cranked locking lever is used as shown in the fist photo. This allows sufficient movement of the clamp in the unlocked position, otherwise, if a straight lever was used, the clamp would have to be placed closer to the bulkhead and would hit it when the opening rod was pulled.