Hi Kris
I like wooden tubes. I usually make octagons but a square one would be fine. In a refractor,you can use the baffles to help support the structure if you wish and keep the weight down using a thinner ply. I haven't made refractors just reflectors but there are wooden tube refractor examples on this forum.
I just did a quick search and came across this example:
http://www.cloudynights.com/topic/48...ood-refractor/
Rod