I have piano lessons in a room on my university campus and the bench provided is a little low for me. My teacher has helpfully suggested books or foam jigsaw mats. Before I buy the mats or carry around potentially heavy books, does anyone have any other cheap, portable tips for raising the height?
Look for cushion foam at craft stores. It usually comes in varying thicknesses, so you might try a couple of pieces of that. Stuff them in a pillow case. Do you meet for lessons in your teacher's office? Whatever solution you come up with, I'd ask to leave it in the office. Or, get a locker which is available to most students at music schools for storing instruments and such.
I carry four telephone books in the trunk of my car. Do they even publish those things anymore?
This is just my personal preference but, I wouldn't want anything that would conform to my butt cheeks. Depending on your size, you may need to lean forward or from side to side and you need a solid base to support your shifting of weight. It seems to me that anything padded will deter that movement.
Personally, I'd consider either asking the university folks to purchase a bench that is adjustable in height ( they're not that expensive, and others learners may be having the same problems as you are,) or even purchasing one myself and donating it to the department and taking a tax write off. It probably won't hurt to ask.