Felted Bag Question

I would like to recycle some wool sweaters into felted bags, and thought I'd ask you all for advice.  The websites I've come across seem to say you should felt the sweater first and then cut and sew the bag.  However, when knitting, you knit it, sew the pieces together with yarn, and then felt it, making for Herculean-strength seams.  Is it possible to cut and sew a sweater into a large bag shape using yarn and then felt it?  Or does it turn out odd-shaped?  Thanks for any tips . . .