Thin conductors on both sides of a thin insulator are connected by wires to a remote switch. Slide it between the batteries. It breaks the connection until the switch is turned on.
Step 1: Why?
In this case, the battery powered loud and obnoxious talking halloween skull was the face of Leaf Boy, a leaf stuffed guy. Too much trouble to get to the power switch, so he needed a power switch on a wire.
Also it would make him shut the heck up until someone was in the right spot for him to yell at them.
Step 2: Materials
Thin. Thin thin thin. It's surprising how little batteries will slide apart sometimes in a battery stack. (Usually a non-springy contact on the plus side and a spring on the minus side) So 2 coins and a popcicle stick aren't likely to fit.
I found some thin 2-sided circuit board material. (thin copper on each side of thin fiberglass board. But I don't know where that is now, so...
Or use paper or thin plastic as the insulator and copper tape or thin copper sheet for the conductors. I think stained glass people use copper tape to solder over.
Then some 2 conductor insulated wire.
Soldering iron, solder, wire stripper, scissors (but not the 'good scissors'), and glue.
And some kind of switch.
Step 3: Cut, Solder, Do
Cut the insutlator strip.
Cut the conductor strips smaller.
Sandwich together. Glue or stick the conductors on if they have adhesive.
Solder on the wires.
Solder on wires first so the heat doesn't melt stuff.
Step 4: The Switch
Could be any kind of normal old switch.
Or a reed switch
Or a backward reed switch.
Or a mercury switch.
Or a pressure mat switch that turns on when stepped on.
Or a clothes pin with conductors held apart by an insulator that gets pulled out by a tripwire.
Or just bare wires that you touch together.
Step 5: Install and Operate
Slide the batteries apart and drop in the gadget. Usually is more like: FORCE the batteries apart and JAM the gadget in and hope nothing breaks.
Turn on the Battery Powered Device.
Turn on the remote switch, it's ON.
Turn off the remote switch, it's OFF.
Duct tape the wire to the Device for a strain relief because someone is going to trip over the wire and rip out those tiny soldered wires.