I think there are better solutions and weight on the roof, c of g and more holes in the roof (mounting bolts) are the downsides.
Some use water bladders. We do too when needed. Consider wakeboard boat ballast bags. You can get them in all different shapes and sizes. They all plug in easily to a 12V cig lighter pump that you can put whatever length of hose you need to transfer the water as well.
I use our old 400lb sacs. Either 1 or both depending on the need.
I can put an extra 100gal of water on the back seat floor of the truck where it’s low and contained. No setup no fabrication and it all folds up to the size of a bed roll when not needed.
