As having an old cracked-screen smartphone laying around and also, having a project in progress with the necessity of a Li-ion battery, I thought that's a good idea to make some sort of adapter to make use of that battery.
Copper covered board for PCB manufacturing
Soldering iron or station
Two pins (like those from arduino)
Battery from an old phone
Something to cut (I've used a Dremel like cutter)
Sandpaper to get everything even
Sharpie or a pencil to trace where to cut
Ruler to make straight lines :D
Step 1: Cutting Everything
According to your battery size, use the battery as a guide and trace some lines with the pencil or sharpie. Then, use a ruler to trace straight lines and surround the rectangle made using battery dimensions, with another bigger rectangle. Between those two rectangles you have to maintain at least the measurements of the height of the li-ion battery. Then cut everything and don't forget to use the sandpaper if you cannot deliver a streight cutting like myself.
Step 2: Soldering
After sanding all the edges, use something to handle the sides of the box and start soldering in points like I did in the first photo. If you want to check the perpendicularity of the soldered piece, use the battery as a template and ajust the angle if it's needed.
Step 3: The Finished Product
After finishing the box, use the dremel to score the copper coating from where battery terminals are going to be. This step is very important because you have to provide a good separation of the box terminals to avoid short circuit. Then drill some holes and solder the pins used for charging and connecting to other devices.
Step 4: Charging Test
After checking the connections of the box with multimeter, connect the battery inside its new adapter to a li-ion charger module. And that's it... :D Aaaaand...don't forget to vote this project on one of the contests! Thanks! Cheers!