A slat or vane for use in a covering for an architectural opening is suitable for use in either a horizontal or vertical orientation and in a horizontal orientation will not sag or droop, and in a vertical orientation will rotate uniformly along its length when rotated from end thereof. The slat is formed in a tubular configuration and has a base material of glass fibers bonded together in a thermoformable resin so as to maintain the arcuate curvature desired for the tubular slat or vane. The slat can be repeatedly deformed and will always return to its original configuration.