JP, I would say the most important part of a pipe rack would be the spacing between pipes. Too close and only little billiards will fit side by side and the larger pipes won't fit but too far apart uses too much material and just looks dumb and awkward. The 2nd is the spacing between levels, too close and only short pipes will fit, too far and tall ones will fit but, short ones will fall out. Even the manufactured ones haven't got the spacing correct for today's large and midsize pipes so taking the measurements from one will only give you a starting point. I haven't any set plans, I started out with some but, as I went a long they got changed and changed.
Oh, thickness of material, not too thin that it flexes like a bow but, not so thick it looks like a 2x4. Just enough to do the job without overdoing it will make it nice and light to hang and appealing to the eye at the same time.