I too use Therm-a-rest. I use either the Camp Seat or the ProLite Plus Small mattress. The Camp Seat does fit any of the economy seats I've been in. It is thicker than their other models. The added thickness allows one to maintain sufficient thickness while partially deflated to reduce the wobbly feeling for which some have criticized inflatables. I also feel inflatables provide a lot more adjustment and may be superior to a solid memory foam cushion if you tend to lie on an angle against the window.

The ProLite Plus Small mattress is the same width as the Camp Seat and tapers. It can be placed so as to provide the same coverage of the seat cushion and some cushioning for your back as well. It's less discrete than the camp seat. However it is my preferred cushion for lie flat business seat such as BA's. I usually carry both in my carry-on. Unlike most in these forums, I don't travel lightly.

I fractured my tailbone a long time ago and to this day it still can be irritated on a long flight in an economy seat. These cushions avoid that becoming a problem.