I just finished interfacing 29 t-shirts and used fusible P44F from JoAnn's, .99 per yard. I used a piece of antique bed sheet as my cover, did a slow sweep left/right and then another slow sweep top/bottom and all are stuck well.
My bolt is the 48 yard one because I will be making 2 memory quilts from my sons' t-shirts, for his two daughters.
Jan