How do you come up with the concept of what you want it to look like?

You simply figure out what you want the stand to do. Is it for transport and field usage. Will you have the plane on a table at the field? When not flying, how much space should it take up in the garage?.
One thing I try to do is to have the stands' footprint smaller than the model.
Four tall legs just doesn't work for me, as it doesn't usually allow for more than one plane to be put into a space like an SUV. I build them so that they are diferent heights so that the planes can be positioned very close together with overlapping horz. stabs or nose to tail, Definately get the PVC cutters.
Do a rough sketch of what you want simply for a parts list of elbows, T's, caps, etc.
They're cheap, so buy a few extra of each. The pipe insulation foam works great. Small zip ties hold them on securely.
Then you just start eyeballin' and cutting. Put all the puzzle pieces together and wahlah! For the wheels, I bought some utility wheels ($6 ea. I believe) from home depot, a steel rod for the axle, and some cap nuts.
The great thing is that it is so cheap that a few mistakes here and there won't make any difference...just cut a new section. Don't PVC weld until it is assembled the way you want it. On some joints, you may want to leave them unwelded for dissasembly. To avoid frustration, start with much more material than you think you will need. Again, it's so innexpensive.

Paul