To start you have several choices, see paper VK5KLT.
Here my choices.
Starting with the support plate.
First possibility is the simpel way. You need to find some support to bow the loop tube.
Then you have to assemble all parts and fixe it together.
Not the best construction but it held two years outdoor, 4 m above ground level in all weather conditions before it was replaced by a better construction.
Some pictures of the plate.
Second possibility: you need more support, to bow the tube and some welding to do. This construction is much stronger and more suitable.
Third possibility: all weld. If done correctly the strongest and best construction in one piece.
Now we have a choice: coupling loop.....
or a gamma match.
How I fix the capacitor on the loop.
You can do it also as in the last picture above. Just make a saw cut at the end of the loop (inside loop) over a distance of 2 cm.
At the end of the groove cut the tube from the inside to the outside for 2/3 of the diameter.
Then bend the lips open and align. You 'll have the same or even better results.
You need a stepper or dc motor to drive the capacitor. Also a reduction inside or outside the motor, 1 to 2 rpm.
You should also provide the wiring inside the tube for the motor.
To control the capacitor I used, as an experiment, a small dc motor with reduction inside. To power it a simple dc hobby mini train traffo.
For the shack I made a control unit for two loops (80 and 40 m.) with a simpel Arduino board, Adafruit stepper and dc motor driver board, small display , a few switches and a motor for each loop.
I wrote the software with Arduino IDE. It's not profesional but it works.
To tune the antenna I just use the up and down buttons and a SWR bridge.
It works fine as long you remember the frequency.
Working on a new simple basic capacitor controller for two bands, 80 and 40m. band. Using again an Arduino board with a motor controller board from Adafruit. This time using only one large capacitor (12 to 500 pF) and a stepper motor with 7.2 reduction, Step 0.25 °.
Here some pictures of the new display layout.
The software will be available for download here as soon it's finished.