Step 1: Prepare design for laser cutting
I used inkscape, which is a free drawing program that's easy to learn. I started by grabbing the template from ponoko's site, started drawing a rough sketch of what I wanted the final product to look like.
After getting an arch I was happy with, I copied it and then used inkscape's boolean tools to remove rectangles where the crossbars will go.
For making symmetrical pieces, you can draw half the piece, then use inkscape's clone tool. This way, any changes you make to the original will apply to the clone.
A couple ponoko tricks:
* You're charged by the time the laser spends, so you can save money by putting things close together, and combine lines when possible.
* Cutting a material that is twice as thick costs four times as much, so whenever possible use the thin stuff.
* Plan ahead. Getting stuff from ponoko takes at least 3 weeks.