I've made both roll top and drawstring packs. They are both relatively easy to make. For the roll top, you need something stiff, but flexible to hold the shape. I use plastic banding, but have also cut strips out of thin flexible plastic. I don't use velcro, but just add grosgrain webbing to the tops and use side release buckles to hold the top closed. I find that I use roll tops on large packs and drawstrings on daypacks. I don't generally worry about compression on my daypacks, so the drawstring is more convenient. I prefer simple construction for my large packs and roll tops make volume adjustment easier. My simplest large pack is a dry bag sandwiched between an external frame and a compression panel.