I have a lot of questions:

Should I use metal, plastic, or wood? How would I make it weatherproof? How would I form the metal?

Any of you who have built fixtures, please advise me!

There's lots to consider when building a fixture and the possibilities are endless.
When choosing materials, you need to consider the type of lamp you intend to use and make sure you use materials that are designed to withstand the temperatures of the lamp so that nothing can melt and create a potential fire hazard.

Double check all the wiring to make sure everything is wired up correctly and if using wire salvaged from other fixtures, thoroughly check all the insulation for any signs of damage and ensure it's rated to carry the voltages that will be present throughout the fixture. This is very important with ignitors as they output a short jolt of high voltage to strike the lamp. Also ensure that the fixture and it's gear are properly earthed and that all wire connections are secure.

If you want a weatherproof fixture, you could always build something inside an empty weatherproof fixture housing. Just be sure to check all the seals and replace them if needed.

If you plan to mount the fixture, be sure that the mount is able to safely support the weight of the housing and everything inside. The last thing you want is for the fixture to fall down and all your hard work becomes a pile of shattered glass and twisted metal.

When testing the fixture, I recommend using a lamp that you're not worried about damaging if something was to go wrong such as a ballast failure. I always test my fixtures on an RCD (Residual Current Detector) protected circuit for additional safety.

The most important thing to remember is if you don't feel comfortable doing something, don't do it.

Below are some examples of how I've made parts for my homemade fixtures
Here I've used curtain rod brackets as mounts for a fluorescent lampholder and starter socket. These brackets are very handy for building fixtures and I've used them for quite a few things.

Here I've used parts from a children's metal construction kit to make a bracket for a pulse rated B22 lampholder that I used for converting a floodlight from 250w MH to 35w SOX.

Here I've used a section of wood from a pallet plank to make a gear tray.

I know this is a long message but hopefully it helps.

Thanks! you have been a lot of help! A way of getting cheap weatherproof housings for homemade fixtures are old halogen work lights and floodlights that use linear halogen lamps. They are already built to take the heat and are weatherproof as well!