Plastic Bag Cap

One simetric tube with thred in each side. One for the cap and the other for the holding ring.
Cut a hole in the corner of the bag a bit bigger than the tube. Then insert the ring around the bag. Next, insert the tube in the hole (you can turn it while you hold the bag around it, so that the thread helps to insert). Fix the tube to the bag with by threading the ring. And finally, thread the cap.