I agree with Roger. Gadgets that seem good always have some downside that makes me stick them in one of the unused gear bins.
For tarps, the best I have found was a heavy duty hair tie. Loop these through the loops, or grommets if you have them, then tie on a short length of line. Thise give several inches of adjustment for staking around rocks, maintain tension pretty well, and even shed ice pretty easily as well as providing some clearance for ventilation.
On packs, a simple truckers hitch works well enough. Some fine cord (say, 1.5mm) saves a tiny amount of weight. Avoid spectra line, it slips too easily, though. Even with gloves on, I manage to get these knots done...