Flick on the switch and enjoy your new light.
You may need to lightly sand down the edges of the resin along the walls (where the top of the mold was). Otherwise it should be completely done.
If you'd like to keep the back covered up you could cut a piece of felt or something similar, cut a hole for the switch and glue the felt along the top edge. Velcro could also work. This would enclose the back, but isn't too important if it'll be close to a wall (like mine is.)
There are an infinite number of appearance and size variations, but the procedure would remain the same.
The method for building up the sides of the mold is one I've been working on for a while. It opens up a wide range of possibilities with resin while avoiding making a 2-part mold (with one part being used to keep the center of your shape open.) It could be valuable on large pieces simply to keep them lighter and use less material.
One other idea I've come up with since posting this is to add a 'glow in the dark' powder in with the cosmic debris. This would allow it to give off a soft glow all the time, even when it's turned off. I haven't tried it, but maybe something like this:http://www.lumi-lt.com/
That would also be an appealing option if you wanted to try the resin casting without all the electrical.