[Gav]

A dedicated aftermarket trans controller is sounding much simpler than the suggestions being produced in this thread!
The other option is just to manualise the trans. Fit a ratchet shifter so you can't select reverse by accident and you can even buy a simple controller for your lockup converter so it only selects above a set speed.
In hindsight I would still fit a TCI or similar controller. Buy it once, do it properly.