Decades ago bikes lacked water bottle cage brazed on fittings. All cages came with straps to hold the cages onto the bike. You can still buy those straps, but they only work with some cages.
https://velo-orange.com/products/bottle-cage-clamp
If esthetics are not important to you, hose clamps also work.
In the photo, I like to carry my spare tube and a few tools in a container in a under downtube cage. The frame did not have the fittings so I used the Velo Orange straps.
But on a hot day I might have a large water bottle down there instead.