GOSSAMER GEAR Seconds Sitlight Camp Seat - is a great multi-purpose item for a great price. 

This sit pad doubles as an back panel padding for GG packs and a comfortable ultralight sit pad. Adding this pad to your pack will help cushion your back.

Available in 3 sizes.

MATERIAL: Closed cell cross-linked EVA polyolefin foam

WEIGHT - Varies slightly  all weights are approximate averages

Small: 50 g / 1.76 oz

Medium: 60 g / 2.11 oz

Large: 75 g  / 2.64 oz

DIMENSIONS

Small:  27 x 43 x 2 cm / 10.75" x 17"

Medium: 27 x 50 x 2 cm / 10.75" x 20"  

Large: 27 x 58 x 2 cm / 10.75" x  23"

NOTE: sizes may vary by up to 1.3 cm / ½ in

Convoluted – ¾ in thickness is a 0.6 cm / ¼ in base plus 1.3 cm / ½ in of convolutions (bumps)