* The header fluid level must be above the thermostat housing (or in this case the temperature sensor as the thermostat is in the return path in a Ford Sigma).
* The rear hood/bonnet slopes away at the back reducing the available height for locating the header tank.
* The air filter is in the top near side in a Sigma as installed in a J15.
* The capacity must be adequate, at least a litre
* It must NOT be mounted on the engine support as vibration could destroy the tank.
The Ford Focus ST MK1 tank is ideal all you need is a support bracket that took 4 hours to fabricate.