I need a new seatback cushion for an armchair--any thoughts?

They say that you can’t have “cheap, fast, and good” you can only have two of the three. I’m looking for “cheap, fast, and it’ll do”.

I have an armchair a friend gave me (friends have given me almost all my furniture, because I’m quite broke). Well, the seat back cushion (that you would lean your back against) has gotten quite out of shape over the years, to the point that it’s pretty uncomfortable. I’m looking for an oversized pillow, basically (about 20 x 30 inches height and length, respectively). My first thought was Ikea, but they don’t seem to have anything. Any tips? Where should I look?

I’m pretty skeptical about making one myself, but if you have a solution I can do in less than an hour for less than $30, without sewing (I like stitch witchery) I’m all ears.

Thanks so much!