These bags are fine, take a 12.5kg bag of sand with a bit of shoving! I wasn't able to fasten the tops once the bags were in, however. The gazebo survived a very windy night, so they work. I have, however, made four more from some old patio covers, so I wouldn't buy these again, as it is so cheap to make your own. Good to have the idea, though, and if you aren't handy with a sewing machine, these do the job. Like others, would have preferred black velcro, the white does stand out a bit too much.
Filled these by adding building sand to carrier bags then dropping them in, I am using them on a small 2.5m gazebo on a patio, they have kept the gazebo firmly in place all weather so far, added bonus is they push the legs to a more vertical postition. Add more sand for a heavier weight. They are literally a canvas bag with velcro straps and handles.