Line your pan with waxed paper or plastic wrap, making sure there's plenty of extra draped over the sides and ends of the pan. This will make it easier (or possible!) to remove the Snickers bars when you're done.
This doesn't go in the oven, so you're just pouring warm chocolate onto the plastic wrap - it wasn't hot enough to melt or even deform the plastic wrap. I would have preferred to use waxed paper, but didn't have any on hand at work when I made the candy bars!
Aside from this not actually entering the oven, plastic wrap is also incredibly resilient to heat. Many caterers use plastic wrap to cover their steam trays to either keep food warm or actually steam veggies. I've even seen one caterer use it to cover scalloped potatoes before he put them in the oven. Granted, the wrap is not actually touching the food in this case, but it does show how high of a melting point it has. Warm chocolate is no problem. :)
Waxed paper would be my preferred method, if I'd had any at hand! Nothing inedible there, and the whole thing lifts out easily.