The Newt design apps are primarily for visual use, and imaging newts typically have larger than normal secondaries to provide full illumination of the camera sensor.
Regarding the ring material, absolutely marine plywood is the best choice. Make sure you allow the internal diametre of the ring to accomodate the mirror, and a small increase as you move forward; allow about 2.5 degrees at the front. A Ray Diagram is essential (I found) and using bright brick layers string to simulate rays on you mocked up truss tube is a good way to work things out. Build your truss tube on a jig (or strongback as the boat builders call it) - essentially an absolutely straight backbone to setup the parts to ensure correct alignment.
When setting up your secondary placement, I found it easiest to allow the spider holder ring to be able to slide on the truss structure, this allows you to accurately determine the focuser and focal point placement.
