G'day Andrew,

We have this info in the specs, but it's easily missed!
The Sea to Summit Large Air Chair fits Sea to Summit Large mats, so yes, the Ether Light XT you've linked to will fit.

G'day Tim,

Yes, the Ultralight Insulated Mat - Large fits in the Air Chair Large.

G'day Tristan,

Yes, the Air Chair Large is suitable for the Large Ether Light XT.

G'day Paul,

Yes, the Air Chair Large will fit the Ether Light XT Large model.

G'day Sam,

The Air Chairs are designed to work with Sea to Summit air-filled mats, and unfortunately based on feedback from people trying this out with other self-inflating mats they don't quite work with the support structures to provide an adequate chair.

G'day Peter, 

I've just checked with the info provided by Sea to Summit, and it says that the Air Chair fits the large Sea to Summit mats - so that would include the comfort plus large. 

G'day Cheryl, 

The Large Air Chair is made to fit large sized Sea to Summit Mats which have similar dimensions to the Exped Sim 3.8 LW (197L x 65W cm) - so theoretically you should be able to use your Exped with this chair kit - but, keep in mind that they are going to be ideally suited to Sea to Summit mats in terms of fit.