Third picture shows innards - battery cover and small switch, wiring, and dental bands for lock spring. Fifth picture breaks down printed parts in detail.
Pictured prints are on Form1 using grey resin for body, and clear resin for glowing eye/secret lock.
The "insert" part is a battery cover for 4 button cells, which screws onto the main body with an m3 machine screw and nut. You'll need to use foil to bridge and connect the cells - and you'll also need to solder in wires and a switch. The switch can be seen on the right in the base of the third image - I used a switch from a old mouse.
It would be a significant improvement to link the switch the eye hinge/lock instead.