Here is a simplistic, but very effective way to judge a reflector: Look at a bulb, which is mounted in your light fixture. Do you see just the bulb, or do you have to look carefully to pick out the real bulb from the reflections of the bulb from the reflector? Each reflection of a bulb is the same as another bulb, as far as how much light you are getting is concerned. Think about it. When you see a reflection of the bulb you are seeing light from another surface of the bulb, so it is light that would not be present if you didn't have the reflector.
If you have a reflector which shows you a full image of the bulb on each side of the bulb, that would be the same as having 2 more bulbs installed, and no reflector. (Not quite the same, but just assume it is the same.) And, if you see two full reflections of the bulb, on each side of the real bulb (not partial reflections, but full reflections) that is the same as having 4 additional bulbs installed.
So, the ideal reflector is one that lets you see multiple reflections of the bulb alongside the real bulb.
I did some testing with a simple sheet aluminum reflector shaped like \_/, with the bulb in the middle. I bent the side "flaps" so I could see 3 bulbs instead of just one, a reflection on each side of the bulb. This reflector doubled the PAR I measured with that light. I did this with more than one type of bulb, too.
Look at the AH Supply reflector:
This is shaped to give 2 reflections of the bulb on each side of the bulb (but only partial reflections because of the dual tube type of bulb) It is one of the best reflectors you can buy.
So, to make a good DIY reflector, just use aluminum flashing material - thin aluminum sheet, which is shiny. Bend it, using a couple of boards and a clamp or two, to a \_/ shape, adjust the angles of the bends so when you look at the bulb, you see 3 bulbs, not just one, and you have a good reflector. Polish that aluminum as well as you can so it has a mirror finish, and you have a better reflector.