In the beginning I sketched, no calculations-just cut 3 pieces of cardboard, which in form resembled the handle detector. Then cut out the holes for the battery and the switch. Then glued all the pieces of cardboard with wood glue and left to dry for 12 hours (overnight). When the glue dryed, I made a hole in the handle next to the switch so that the wires could freely pass through the hole. Well, finally, you need to glue the reel to the handle of the cardboard. I used glue for hot gluing.