There are two options for installation: Into the engine compartment or the trunk.
Of course, each location has advantages and disadvantages. The HHO kit should be located within the vehicle's body. Wind, sand, heat, direct sun rays, stones, and other elements must be avoided. Examine the area around the engine. Most of the time, you cannot see it visually. To inspect it, you may need to remove some parts, such as plastic covers over the engine, front bumper, or plastic inner fender above some wheels. There might be some room.
As an alternative, you could consider mounting it in the trunk. Because of the charging system, it is preferable to be in front. All wires should be as short as possible. When you have to do the wiring, it will be much easier for you.
However, there will be extremely long wires running from the front to the back of the car. I do not recommend it because of the possibility of the cables breaking, which must be secured! If it's in the back, it's taking up space in your trunk. There is no ventilation in the event of a gas leak. The gas will be stored in a vault that is not secure.