I love your bag. I have been experimenting with plastic bag yarn myself. Besides bags, rugs and placemats do you have any other suggestions? I am currently trying to weave mine with an old hemp yarn. Please let me know any other ideas you may have. I am very interested.
You could try to make a box. You'd have to knit/crochet it pretty tight though so the sides would be stiffer, which would be pretty uncomfortable to work on. I also saw some article about a lady that would braid plastic bags into rope then make sun hats out of them. I guess they sold, but I think that would be very sweaty to wear on your head.
Anyway, that's a great plastic bag bag. I recently knitted a tote bag with crocheted handles and was surprised how awesome it turned out. How many plastic bags did yours take? I swear mine took at least 40.