The Boxes are made to measure, therefore you can have it to your exact specifications. These are intended for use with many types of conventional gas fires and Live Fuel Effect fires.
These should only be measured and installed by a qualified person.
Liners help contain the combustion products created whilst the fire is on, it also helps direct the combustion products outside, they also protect the chimney walls from heat and corrosion. These should only be measured and installed by a qualified person.