Believe it or not, those tubes they roll rugs on make great bass traps when installed in corners, and they are free. Just go anywhere rugs are sold and ask for them.
Bass trap in a difficult corner
I have a corner that has very little space for a bass trap. On that wall is a large book end which I think makes for a decent diffuser but there is still a corner right next to the bookshelf that is completely exposed. On the adjacent corner is a trap. Will that be enough?