There's a full video of the battery build here.

My plan was to mount the battery slung under the frame near the seating area.

But before I could commit to this I needed to mount the seat. I planned to use a simple go kart bucket seat from Aliexpress. The dimensions for this were well defined so I created a seat mounting plate that would also allow for some adjustment.

I cut this on the ArcDroid.

and created some mounting bars that fit to the frame. These had M8 welded studs to match the adjustment slots.

With the seat mounted I turned to the battery box. The batteries I chose are 60V 35Ah LiPo. One for each motor. These come with integrated battery management so not much to do other than box them up.

Using some cardboard I created a mock up of the box and taped it in position on the frame. A quick test convinced me this position would be ok.

The box is made from 1,5mm steel plate with a steel frame made from 10mm angle iron. The top plate is removable for access.

First I cut the angle iron on the mitre saw. This is a normal saw designed for wood but with a TCT blade fitted and a speed controller. It works pretty well and saves me having to have two different saws.

I TIG welded the frames together.

And plasma cut the steel plates.

I also used the plasma cutter to add the holes for the panel mount RJ60 connectors for the battery terminals. These are to be bolted in place. Rather than mess around getting it to cut the small bolt holes, I'll just drill these later.

Then the box was tack welded together.

The box is suspended from the frame by some steel brackets. I created the bends in these on the bending machine at Hackspace Manchester.

They have a slight angle on them to give some clearance from the box lid so I can easily remove it.

A quick test fit showed everything was OK. 

They were then trimmed to size and bolt holes drilled through the brackets and the frame.

I painted the box black and for show put some yellow diagonals on it with tape. I used wide masking tape to get the spacing right.

Overall, I'm please with the way this turned out. I silicone sealed the box. The batteries are encased in foam within the box for insulation and I alo added some internal resettable fuses, just to be safe.

There's a full video of the build :