The mouth will be in the front firing towards the floor, just follow the front firing option towards the back of the plans or follow the end firing directions and aim the mouth upwards.
If the budget is tight look at this driver. <Here> Its perfect for the slim design and saves you some cash.
Try to purchase an amp with DSP options such as <this> for best results out of those horns. No need for big power in these designs, they are super efficient.
If you have any more questions or need help along the way i will be back after the new years parties are over.