The shape and the configuration of the fitting lift bag will depend on your flotation needs and dive environment. As for the construction, there are open (parachute) lift bags and closed lift bags, or as they are also called camels.
The open bottom lift bags are filled with air from the top and displace water out of the bottom. Once the bag is buoyant enough to lift the attached object, it will start to float to the surface. If a bag is partially filled, it will accelerate as it ascends due to the air expanding at reduced pressure. A full bag will overflow or blow off excess volume through the relief valves. The shape of an open lifting bag is most often designed to distribute the volume in a vertical rather than a horizontal direction so that the open end of the bag always remains underwater.
Closed lift bags are slightly less volatile but come in a wide range of sizes with different lift capacities.
It is important to select a lift bag with an appropriate capacity for the task at hand. If the bag is too small, it may fail to lift the desired object, whereas the grossly oversized bag may cause a runaway or out of control ascent. Most modern lifting bags incorporate dump valves to enable the diver to control the bags buoyancy during ascent, so pay attention to this feature when making your choice.