Warren, I managed to squeeze a Downmat into a chair kit, but even with minimal inflation I popped something on the pad. It wouldn't stay inflated any more. It could've just been a fluke, but IME I'd consider another option for your chair.
Given that the Trekker chair kit weighs 10.5oz, perhaps a Crazy Creek Hexalite at 14.8oz? Supplement insulation w/a small square of foam on the seat?