Today, I welded a frame to hold the batteries. Batteries can now easily be mounted onto the kart, and swapped with new packs (we'll get more, new batteries tomorrow) when they run out of juice.
The biggest/loudest/hardest/most daunting parts are now finished. We still need to mount a sprocket on the rear axle (using standard kart parts), align the motor with the rear axle, and get some electronics and cabling on the kart to actually be able to drive it.
The finish is in sight, and I finished all the jobs that I wanted to do this week. Excellent!