I've used 3.5mm fiberglass tubes for the kites that are shown on the videos. The spreader length is 1m. The spreader is bowed using a tension line.
For my other gliders I use carbon tubes, fiberglass rods and even bamboo. I think that the ideal combination is a carbon spine and a fiberglass spreader. A greater weight of fiberglass is not a problem for a spreader because gliders require a greater weight on the nose.
Gliders that are made of a paper, bamboo and cotton line are my other favorites
I'm looking for an "ideal" sail material for gliders. Fabrics, plain polymer films, tyvek all have some disadvantages.