I also gutted one. Drained and Cut the top off neat.
Then I drilled and taped the post I can go 2 ways that way, to hook up cables. I can use the washers and post as shown, and use a top post terminal.
Or a cable end with washer and bolt.
I did detail the plastic box. like you would do a yellowed head lamp.
And added some angle plastic I had with epoxy to the lid to keep it in place. Looks like angle iron but is plastic




As you can see with the red top caps it was not yet 100% polished up yet