To clarify, the sand I use is "tube sand". The bags are quite narrow and pretty stout. I usually shake them down vertically to compact the sand and fold over and duct tape down any excess bag material. This makes for a solid tube shapes weight that straps to a frame really well. I will usually wrap most of the bag with duct tape to reinforce it and always wrap where compression straps rub or where additional weight will contact it. Then I support it with the grab-it and run 3 or so straps around it. No bounce, slippage, or movement at all.
When I do stairs, I decrease the weight to sAve on the knees and increase the intensity to burn the muscles. I also alternate between hitting every step, to skipping every other step to tax different muscles and better mimic climbing etc.
I will say, however, that there is no substitute for hiking actual hills, mountains, etc. Nothing in the modern, "easy" world will beat you up quite like a loaded pack on a mountain trail. The muscles are there, the heart and lungs are there, but "the mountain has it's own ways" as a fella once said.