I had a look around, on the Internet of course, about home make DIY antenna to improve performance of my router.
This one from down under is a good resource and I think I can do one in paper first. A simple one such as a Corner Reflector can also be done. You can do it by card board or even a CD case.
Commercial reflectors are all in parabolic shape, you really have to know a bit of math to do a perfect shape. Check it out here at Wiki.