I took my chess piece Knight design and removed the base,  split it in half and introduced a 3V Button Cell, a 5 mm white LED and a push button to create a keyring torch.  I have included STL files of the two halves but you must mirror one of the sides in your slicer for them to face each other.  Most slicers have this option.  I printed the body at 102% and the button at 100% and they fitted well.   Print the sides flat with support.  For assembly make a paper circle 20mm dia with a 5 mm hole in the centre. Take the side with the hole for the button and insert the button from inside.  Take the LED and slide the long leg +ve through the hole and onto the button.  I sanded the lip off the LED to help.  Place the paper disc over the LED leg so you can see the leg through the hole in the paper.  Place the button cell +ve side down onto the paper.  Now carefully slide the other side of the head onto the other LED leg through its hole and align the 2 sides.  Check the action of the button that it is loose and rebounds when pushed and that the LED light comes on when pressed.  When happy run some liquid superglue along the joint.   Button cell is CR2032 3V 20mm dia.