I cut the top edge off a large yoghurt container. You know how they often have a 1/2o1 inch lip? I snipped it at one spot to give me an opening. Then I made a casing with a bit of extra room and threaded the plastic ring through it.
It has help up for about 5 years now. I did not try to secure the loop of plastic or stitch it closed. I did stitch the casing closed.
I used sand from the dollar store for the weight. I put it in a plastic baggy before I sewed it into the cushion part. Again it has held up for quite a long time.