In selecting the battery, I was looking for something that was compact enough to fit in the narrow deck frame, but high-powered enough to make it up hills and last for miles. Lithium ion batteries are somewhat expensive, but have a high enough energy density to satisfy my target specs for speed and power. Here are the ones that I opted for:

https://hobbyking.com/en_us/turnigy-high-capacity-10000mah-6s-12c-multi-rotor-lipo-pack-w-xt90.html

There are two of these batteries included in the deck assembly for a single scooter.

If this scooter were to be produced on a larger scale, I would purchase Lithium Ion 18650 cells and attempt to produce a custom pack. This form factor is cheaper than the rectangular batteries, and because of its popularity, provides more options for customization and replacement.