Check out Reefkoi website. He makes a couple different ones. Not sure if its cheaper doing it DIY but from looking at them at the shows it seems like a pretty simple design.
I used t slots 1" extruded aluminum. I first used it to create a stand for my metal halide fixture. I later added vho to the same fixture and also an led bar. I took it apart and now it's being used to hold my led fixtures up. I like how it is very modular and can adapt to many different kinds of fixtures.
I used a hacksaw to cut it but I'd recommend a miter saw with a blade specifically for aluminum to make cleaner and more precise edges. Be careful and read up on that though cause I heard it is dangerous when the shrapnel bits fly out.