I have had, and am still having, the same problem. I have been going through about 3L of coolant per week at its peak but I am nearly on top of it now. My first problem was that the paper gasket between the head and thermostat housing wasn't sealing to well. When the car was hot you could open the bonnet and see bubbles as well as hear a sizzling noise as the coolant touched the head and evaporated. That was fixed with a metal gasket and gasket sealant, now there is no more leak from there. Other leak is from between the thermostat housing cover and housing itself. Having used a sealant between the two it only seals for maybe a day until the coolant makes its way out. When the engine is hot there is no visible leak, but when cold I used a cloth to wipe up the coolant and watched where it was coming out from. I believe it is because the thermostat housing cover is not flat so have got a new one and got a new radiator cap to make sure that isn't a problem too. That should fix my problems I hope.
1992 Ford Falcon EB2 GLi sedan I6 A4
Fairmont dash, Pioneer DEH-P5550MP, Trip Computer, XH Snorkel, EL Thermo Fans