This is a simple conversion of an old 2-cell D-size Maglite flashlight to LED-lamp and 2 18650 Li-Ion batteries (7,4 V).
Printing a very simple distance holder for the thinner (but longer) 18650 batteries is step 1 of 3.
Step 2: Providing a conversion LED lamp for the P13.5S-socket, taking 7,4V (of 2x 18650 batteries). I found an affordable one with 3W CREE-LED for 4-12V at AliExpress. Search with these parameters: P13.5S LED conversion.
This one just fits into the Maglite.
Step 3: Modify the tail cover of the maglite. Since 18650 batteries are longer than D-size cells the spring in the tail of the flashlight has to go IN the tail cover instead of onto it. See pictures. Use a vice to bend the spring tighter. Make sure it has contact to the minus-pole of the batteries. If your Maglite is colored then remove the color under the spring to make contact. Use a meter to make sure everything has electrical contact.
Of course you also need 2 18650-type Li-Ion batteries. Take care that they have a proper plus-pole tip (not too flat to make contact inside the Maglite). And remember that you also need any charger for the batteries.
This conversion will give you a dramatically much brighter flashlight than before (2 last pictures to compare). But since the reflector is not optimal it won't a such a good one as a genuine LED-flashlight.
Inner diameter of this part was much less than defined - because of warpage I assume.
Better test after some 2 or 3 centimeters printing if 18650 fits in.