Insert the battery with positive facing up (again, carefully, these retainers are a bit flimsy, especially after our modifications)

The retainer my look a bit bent with the battery installed, this is okay as long as you're careful in handling it. Too much rough handling may result in ripping the contact pad off of the cart.

Place the motherboard back in the cart casing and place the top lid back on. Make sure you don't need to press down too hard. If you do, you may need to remelt the solder again or do some more sanding (remove the battery before either)

If you can slide the top lid up without too much resistance, you're good to go!

Gently screw the tri-wing screw back in and test your cart!