My son bought a X-Craft Hovercraft (http://www.rcmania.com/fastlane-rc-x-craft-hovercraft). It was cheap (end of life) and announced with a “12v power” where a lot of similar products were 9.6v.
Indeed, 12v is really a plus and the hovercraft is pretty fast. Unfortunately, the battery is a good old NiMH rated at… 600 mAh (10 x 1.2v cells, near half aaa in size) ! It last between 8 and 10 minutes for a charging time of around 5 hours. It was difficult to find another one at a reasonable price and the charging time had to be monitored carefully (no auto off).
So, I decided to build two 12v LiPo (11.1v to be exact) with three 18650 cells. I had to change the way the battery could be plugged on the Hovercraft.
I also used a charger I already had for heated gloves, which was able to charge two 12v LiPo at the same time.
The final result : 30 minutes of play time with one battery, 4h to recharge both with an auto stop ability (mandatory for LiPo batteries).
Used materials for each battery: 
- 3 x NCR18650B batteries
- 1 x 3S LiPo PCB protection board with balance function
- 1 x 12v round connectors
- 3 x solder tabs
- 1 x DC connector (5.5x2.1 mm)
- Some adhesive tape
Soldering cells in series and wire the 3S protective board:
Attaching the DC male connector and finish with the adhesive tape:
Soldering the female connector to the Hovercraft and plug the battery
