The design of your FirePit is entirely up to you. There are certain size restraints to ensure the burner can be installed but, apart from that, the overall look is left to your own imagination. You can build it using practically any brick or concrete non combustible products that match your existing garden design.
One very important feature of the FirePit design is that it must have some form of ventilation holes around the base of the pit at ground level. This can be achieved with normal air-bricks, or just a series of holes drilled through the brickwork. These ventilation holes allow water to escape otherwise your pit would fill up when it rains. This also allows air to enter underneath the burner, which is required to maintain the correct gas/ air combustion mixture. Finally, LPG gas is heavier than air, which means any unburnt gas will sink to the base of your pit and, without a means to escape, would build up to a dangerous level. (Natural gas is lighter than air so does not have this problem).