Hello Partridge,
Most gas bottle lockers have several ventilation holes in the floor. It is usually possible to pass a strap through the holes so that it can be also pas over the gas bottle and when buckled, to actually hold the bottle down to the floor.
You can get small 'cargo' straps from most motor spares shops, which are plenty strong enough, and often have a ratchet tensioning system to allow then to be easily adjusted for length and pull down force.