Of all the things you can build yourself, if they are what you are going to use to judge EQ & mixes, I don't recommend building monitors. That would be like a carpenter starting by making his own ruler. Just the room and mics needed to verify accuracy of speakers is just not available to mere mortals unless you are a student at MIT or somewhere with fantastic, calibrated equipment. There are so many manufacturers with low priced, pretty good monitors, you probably can't buy the parts to compete with them even if you had the design.
Monitors are the part of your studio that influences every decision you will make in your studio. You can't even properly judge your room until you have good monitors. If you want to build some for loud sound and fun listening, that makes sense, but if you mix on them, they won't translate.
