Depends on the use you will be doing. I run just 14s 5ah in the rear rack of my dirt bike, and for a longer run put 5 ah more in a handlebar bag.
Just 4.5 pounds on the rear rack, but I won't say you can't feel it. It does not buck me off like when I carried a 15 pound ping in a rear rack.
But I assume you want to carry at least 800wh, and for that the easy thing for me was the saddlebag type boxes. There are various bags though, that can be used. Two handlebar bags for example.
If you dig deep enough, I did show in a thread on the Death Race bike how the boxes were built. I did drill two holes in the frame to mount those boxes to the Y frame dirt bike later. I never developed a crack in the frame, despite riding hard enough to shake parts off the bike several times.
It's really easy to build the boxes with simple tools, aluminum or other sheet material cuts with a skill saw, and the thin wooden sides of the box hold screws easy. No welding, no needing a metal brake to bend sheets.
You can shape some wooden blocks, to go under the box, if you want to do a clamp on rather than drill the frame, Curved wood facing the frame, flat wood on the box. Then bolts to clamp it in place. Notches as needed for cabling, or re route through sheathed cable.
