It has to be aluminium rod. You can either have the tube connecting to the centre of the PL259 or the rod can connect there. The other section moves and is clamped to the driven element which goes to the ground connection of the PL259. You are basically making a variable capacitor to be able to tune the antennas impedance to the impedance of the feedline being used. If using open wire feedline you will also need the relevant balun to convert the impedance back to 50 Ohms at the tuner or receiver. If you run coax up to the antenna you just need to match at the feedpoint with a balun or with your gamma match. If the antenna has an impedance below 50 ohms you will need to add resistance not capacitance. If using open wire you can also try tuning with a Tee match or a Delta match. In all 3 cases the driven element can be one length and does not need to be broken / open at the usual feed point. Some people find it strange to see a short but we are talking about AC not DC. Look up delta gamma and Tee tuning / matching if you are interested in matching and or open wire feed. Gamma also can work with coax matching. Not sure about Tee tuning and Delta but there is probably a way to do it if you convert from coax and go into the Tee or Delta match network. I have used gamma match with coaxial fed antenna before and can work very well but you must be aware of water getting into the gamma match. Once adjusted you can use self amalgamating tape but wind it from the bottom upwards, i then cover it with insulation tape wound in the same direction to help keep the uv off then i cover those in heat shrink to stop the tape unwinding and further protect against UV. You can also put a little silicone or similar in the heatshrink just before you shrink it. Drainage holes are not effective as the water can get right up between the dielectric insulator. The larger the rod used the better for transmit. Experimentation and research can help you find the best sizing and positioning for your gamma match.