VEVOR Plug in Hanging Light Cord Kit, 16ft Lantern Extension Cords, 2 Pack E26 Light Socket Lamp Cord Replacement, Secure & Sturdy, Easy to Install, Pendant Lamp Kit On/Off Switch & Socket Rings

I would recommend using low voltage bulbs in this sort of application, especially considering paper is easily combustible. If you insist on continuing with 120V lamps, you need to change the sockets to fully-enclosed ones. Topballerina's comment about pendants triggered this thought- perhaps use a premade socket with cord intended for pendant lights. The threaded part of the socket and mounting ring intended for a shade could be used to secure the socket in a properly sized hole in your art piece. Example (I wouldn't use this particular one because it doesn't say it is UL or ETL listed, but this is the concept.)