In his mail Geri, DK8KW, has given a very good description of what to do.
I would like just to add 2 options :
1. If it is mechanically possible you might gain some dB's by placing the
loading coil (or at least a part of it) at the top of the vertical section.
Especially with a rather small top-hat as you described this will increase
the effeciency of the antenna. I believe that G3XDV has some experience
2. Having several parallel wires in the top-hat will increase the antenna
capacitance and is in some cases mechanicaly simple to implement. I use an
inverted-L antenna with a 13m high vertical section and a 26m long
horizontal section that consists of 4 parallel wires each 0.8m separated.
73, Rik ON7YD