Just use the negative regulator in the negative line:
View attachment 19510
And it's very important to remember that the 'Positive' side will be 'common'
*You may know that most cars have the negative of the battery tied to the chassis. This is the 'common' for those vehicles.
Some cars however use a positive 'common' which can add some confusion. For example, when installing a stereo the case is designed to be used in a common negative setup so in order to properly install it you need to make sure that the case does not touch the chassis of the car or it will short out.
So. If this project is to be used in an automotive situation where the negative is connected to the chassis, you need to make sure that the negative from your regulator never touches the chassis or any other negative.
Keep us posted and we can give more details.