Designing an over voltage protection for really low voltage
Too long for a comment.

The camera can use NiMH batteries so two in series (check that they are in series - there will be a simple metal piece that connects + to - when they are installed) is 2.4 volts. Current won't matter as long as there is enough. It won't take any more than it needs. You can use the adjustable and set between 2.4 and 3 volts. Or use a cheap 5V wall wart and put 3 diodes (like 1N4001) in series on the + side of the supply. Silicon diodes have a drop of about 0.7 volts so 3 will drop 2.1V of the 5V leaving 2.9 for the camera. If you build the diodes into your plastic or wood battery substitute and use a connector to the power supply, you will have protection against reverse power connection.