Decades ago before water bottle braze on fittings on frames were common, water bottle cages came with straps to hold them on the frame tubes. But the design of those straps makes them only work with some cages.
VO Bottle Cage Clamp - Water Bottles & Cages - Accessories

Hose clamps would look uglier, but would function just as well and might be cheaper.

I wanted to add a bottle cage (for my tools and spare tube) under the down tube on teh bike I built up this past winter. See photo.



But you certainly could use them to mount a bottle cage above the down tube in a different spot.