A super duper easy pattern would be to cast on, o let's say, 50 stitches, knit every row until you have a piece that measure twice the depth of your desired bag, then bind off, fold it in half and seem the two sides. then you can either make I-cord, braided, crocheted or purchase some handles and attach them. and POOF
ya got yerself a really simple bag!!!
You could also adjust where you fold it so that you could have a flap on the top too.