Depending on your weight and the windforce you will need an larger/smaller or even no bag at all. Solo sailing (± 95 kg incl. suit/trapeze etc) with 5 beaufort I don't need a bag at all. As new sailer however you'll have to deal with the righting in lighter conditions. I use an ordinary shopping bag as often can be bought in a supermarket, contents aprox 40 ltr. With less wind I have to fill the bag carefully and completely. To avoid water leaking out I even added a plastic bag inside (sewing machine). At the end of my righting line I made an eye. After having filled the bag completely I can just lift it out of the water and put it on the lower hull. I use a carabiner or just a knot to connect bag to line. Then I get into the position myself with the righting line over my shoulder. By strechting my body (legs/bag) I'll lift the bag just out the water having max torque to turn my H16 upright. Don't forget your bag afterwards. Your H16 will be dead slow otherwise...