Challenging … Need many big and firm (inflatable?) cushions
* raise seat by 5 cm so my neck can lean back
* lower-back, mid-back

Firm means bulky…

Need to test somewhere.

If effective, then I could possibly doze off while sitting upright

Neck curling-in is a bigger problem than crammed legroom . legroom can be comfortable, but ranks low in terms of sleep deprivation relief:

* sleep before long flight
* find 3 or 2 empty seats
* convex
* legroom