Maybe.
This is probably a common problem, because the water bottle mounts on my mountain bike are in the exact same place as the bike I bought to convert to e. So aren't there standard solutions?
I have seen a couple adapters, e.g. https://www.amazon.com/Ebike-Conversion-Battery-Adapter-Aluminum/dp/B094QV6SF6/ and https://www.amazon.com/Accolmile-Bike-Battery-Base-Bottle/dp/B09NQ3P7MG.
Also https://lunacycle.com/no-drill-battery-mount-kit/.
Except for the Accolmile kit, it looks like these would allow a thief to steal the battery just by undoing the hose clamps.
But I was wondering if anyone has thoughts about what to do. I could mount the battery with the tapered side down, and it looks like it would fit that way. And just keep it locked so the battery can't slide off.