Well, you could start by surveying the commercial offerings in those cases where they have some amount of experience in the marketplace, and in particular, focussing on those aspects pertaining to the operational parameters. Then a few things stand out such as: a) electronic control of a solenoid *mixing* valve; b) low voltage 12VAC operation; c) doubly insulated, waterproof, and corrosion proof housing all around; d) display timers indicating time remaining to the user with maybe a 30 second beeper; e) usual array of ground fault detection and other safety features.
The electrical feed should be in the ceiling above the shower, with a waterproof conduit connection to the valve housing through another waterproof fitting. The plumbing uses as much of existing as possible, preferably requiring only threaded connections. This isn't going to be cheap. The OP should just buy a finished product.