For a photo, I'd try pipe insulation (white foam, not the black foam) with LED rope inside.

     To maintain shape, you might use coat hanger wire inside, or lay the shape out on the floor and fill the void space between LED rope and foam tubing with resin, letting that set up thoroughly before handling the outline.  Downside would be that you'd not be able to re-use the materials for another shape.

     The LED glow through the foam would be attenuated enough that you'll probably need to use a tripod and long exposure to get that neon look.

Poser 12, in feet.  

OSes:  Win7Prox64, Win7Ultx64

Silo Pro 2.5.6 64bit, Vue Infinite 2014.7, Genetica 4.0 Studio, UV Mapper Pro, UV Layout Pro, PhotoImpact X3, GIF Animator 5