Synthetic what ever I've got. Mostly I have adapted sleeping bags to an underquilt. I am currently muddling through a KAQ design using an old comforter I picked up for $5. Wally World fabric.... Like I say I can live with the weight penalty and I don't go long distances partly because I can't. But the compression ratio stinks.
Anyway, if you need cheap... there are ways of doing it, but the penalty is there... no question about it.