Seat cushions for flights

I used something very similar for my flight to Europe last November, after hurting my tailbone in a car accident. It made a HUGE difference on a 10 hr flight. I suggest ordering it in advance and trying it out. Some are made of memory foam and very firm/not comfortable. The one I used was from Walmart and was just regular foam. Watch the width of the cushion compared to the width of the plane seat.

I used a foldable duffle as my personal item and carried the cushion on the plane in the duffle, along with a few other items.

My husband uses xtreme comfort coccyx seat cushion. He has used it for two trips to Germany, seemed to help.
It is made of memory foam. We put it in a carry on rolling suitcase.

Depending on your airline, you might just be able to strap or attach the pillow to the outside of your bag, like you would a neck pillow, and not have to worry about making room for it in your bag.

You could try a self-inflating sit pad:
https://www.rei.com/product/845302/rei-co-op-sit-pad

Packed Size 3.5 x 8 inches

Dimensions 16.5 x 12 x 1.5 inches

Weight 4.4 ounces

Adjust air fill from soft (partially filled) to hard (filled). Light weight and rolls compact.

I've used seat cushions for long flights for years. Makes a big difference. I'm currently using a gel based pad that folds in half and is very thin - packs easily. I found it on a travel gear website. One thing to remember - don't forget your pad when you're in a hurry to leave the plane! I made that mistake once and had to replace my pad.

We have ones from Amazon we bought years ago after a flight on an old Delta plane where the seat cushions were hard as rocks. It's light and rolls down to fit in its own carry bag. Similar to the rei one.

https://www.amazon.com/TETON-ComfortLite-Self-Inflating-Lightweight-Portable/dp/B009L1MXE0/ref=sr_1_4?dchild=1&keywords=inflatable+seat+cushion&qid=1578921206&sr=8-4

Getting recommendations for a seat cushion is like getting recommendations for shoes. Every, ahem, foot is different. Before you buy get yourself to a store that sells them and give a few a test drive. I bought several on line for my lower back (multiple herniated discs) and until I went to the Back Store I got no relief. But, the one that works is unfortunately not easily packable. So, still searching for that magic combination of effectiveness and packability.

Bad right hip, so I need a seat cushion in cars and on planes. However, I don't like the weight, expense, and size of the real seat cushions you can buy, or the way they feel, so I bought two thick squares of memory foam at a fabric store and cut down an old pillowcase to fit them into. This homemade seat cushion can be rolled up and then compressed into a very small nylon stuff-sack that I usually clip onto my carry-on. This might be enough for you if it's just general "padding" that you lack.

I use an inflatable neck pillow, which I inflate (half, to make it softer) while waiting for take-off. Takes up no room and no weight in my back pack. The hollow for the neck is very comfortable for my "tail".