I presume the filler neck is on top of the tank and sits too high. You can
put a filler cap between the thermostat housing and the inlet tank of the
radiator. I believe you can find a suitable one from one of the usual
aftermarket suppliers. This does not preclude using an overflow tank, which
is pretty much mandatory, especially on a Tiger where you need optimum
cooling efficiency; i.e., no air in the coolant. I assume by "airlocks" you
mean air in the coolant, which is inevitable without an overflow system
because of the expansion of the coolant on heating. I realize it seems that
one could have just the right amount of coolant so as to fill the system at
operating temperature, but the reality isn't so simple. For example, when
you first fill the system it takes several heating/cooling cycles to get
all the air out.
An alternative to putting the filler between the radiator and thermostat
housing is to weld a filler neck on the OUTLET tank. You will need one with
a right angle bend in it so it can be positioned to just clear the hood.
This will not be as high as between_the_radiator_and_thermostat_housing
location, but this is where I have mine and it is high enough. Again, use
an overflow tank, which you can put wherever is convenient and connect it
to the fill neck with a long hose.
Hope this helps,