The Trekker 20 Chair kit converts most of the models of Thermarest that we stock into a comfortable seat, is very strongly made, and weighs only 300g.
Fits all 20" (51cm) wide mattresses, and all short and regular size mattresses.
Comes with a mesh storage sack and compacts down to a package 35cm long by about 7cm diameter which can easily be stored in a rucksack.
NB - does not work without a Thermarest (not included!).