I used cedar from Home Depot, has to be picked thru extensively to get a few boards.
8' 4x4 if possible or 2x4. It has to be scarfed to get the length, I use epoxy for glue. Use about an 8/1 scarf - believe me 2/1 and 4/1 doesn't work, I broke the joints while trying to bend them around the frames.

I would like to try Poplar, it is much tougher but a little heavier than cedar. Still very light compared to most woods. Around here it is cheaper than almost everything, including cedar.