No one mentioned the most important point. Do NOT turn your object over until the paint is absolutely dry. If this means walking away for a night, then do it. You will be glad you did. Allow at least as long as the paint instructions say for a touchable cure, if not longer.
For flat objects like batarangs, I would make sure the objects (coins or washers are good) I use to keep the rear surface off the paint-support-surface do not leave any area out in the line of fire. With some spray paints, the paint may develope a slight skin connecting the left-visible area of the support to the edge of the object being painted leaving you more to clean up before turning over.
For a nice shiny finish on both sides, I would not use the finishing nails sticking out as some paints, especially shiny ones take a long time to cure and may still be soft enough to receive an impression from the nail tip sinking into the soft paint. Again, I'd say refer back to point number one!
In my experience with Rattlecans (and at my job, we go through dozens a week) glossy dries much slower that flat. Auto paints tend to dry a bit faster since no-one wants to leave the car untouched for days while the paint dries.
RGP
I've used cardboard too but, got tired of objects sticking to it. Flat surfaces tend to inhibit consistent coverage where the bottom of your project meets it. Since the air and mist flows through the egg crate, you spend less time and effort getting a really nice finish.
