I've found these to be the most useful and versatile method of fastening bottle cages where no dedicated bosses exist, having also tried Minoura and Topeak alternatives. These SKS ones are well made and robust, and grip very well on regularly shaped tubes, including those that taper (as there are two independent straps).
A tip for those who may need to use them on irregularly shaped tubes that may otherwise suffer from slippage problems...…
I have these fitted to a bike with a tapering top tube whose cross-section is an irregular hexagon with radiused vertices - this profile/configuration means that the straps need to be done up REALLY tight to prevent slippage/rotation, as the mounts are, due to i) the horizontal orientation, ii) the positioning in relation to the vertices, and iii) the constant vibration/jolting on the UK's appalling road surfaces, continually trying to work their way around the vertices and hence out of line on the top tube.
Now, you'd think that just pulling the straps as hard of possible should work.....but it doesn't in this scenario. The way to deal with this is to take advantage of the fact that the Velcro straps expand when wet - a phenomenon that of course actually increases the likelihood of slippage when cycling in rain.
The answer is to thoroughly wet the straps before fitting, allowing them to expand, then fit them, pulling as hard/tight as possible without denting the tube, then as they dry out they shorten/constrict and tighten considerably further. This drastically increases the chances of the mounts staying in place on awkward irregular-profile tubes, as in the dry the straps are super-tight, and when it rains, they loosen only to roughly the equivalent tightness of pulling dry straps as tight as possible.