18650 Battery Sled

The original design is pretty good and functional. Thx to the Maker "Allday"

Here are the changes i made:

- Made it a bit more sturdy in general
- removed some details (+ and -,as i use it for series)
- extruded some regions for easier printing
- removed unwanted edges, that could produce artifacts while printing
- added 0,3mm length in the inner battery-part to release a bit of pressure onto the battery-contacts.

Total sizes:

- 75,30mm length
- 74.30mm width
- 14,10mm height