Here's what I have done, both to McGyver a lumbar support and emergency pillow--carefully roll your clothes into a log about 10 inches across. Roll reasonably tightly. Then insert into a gallon sized zip top bag and push to the bottom. Begin to roll that bag up, pause to push out the air, and zip it shut. You can adapt this to how firm you want it (for a lumbar roll) or squishy you want it (for a pillow, for my taste). For using it as a pillow, I then insert the zip top bag into a T-shirt because I don't care for plastic against my face, but YMMV. Back in my pre-HSR Chinese rail travels, I used this hack frequently.
Honestly, I never travel without an assortment of sizes of zip top bags--they have a multitude of uses! That same gallon zip top bag can magically transform itself into a sink for washing socks and undies, for example. Small baggies can keep food items clean and edible for eating later on, and will prevent your snacks from contaminating your day pack. I know that a lot of folks say duct tape is the more versatile travel item to tuck into your suitcase or backpack, but for me, it's zip top baggies!
