How to raise turtle from egg without incubator

I was walking m' dog and he stops to try and grab something. I stop him before he can get it, it's apparently a turtle egg. It was still warm but when I looked around I didn't see the mother, nest, or other eggs. I took it with me, put it in a warming box filled with dirt and soft mulch. It's raining outside so I have it inside, it's 65-70 degrees. The mulch is warm surrounding it barely covering it and there are big vents to let in air.

Is this warm enough? If not how warm should it be? If I do need a heater where can I find the proper one? Do I need to keep it wet or dry? I don't want to abandon it outside so it dies but is this a lost cause?

The below site should help you with your turtle egg problem, a word of caution, if this happens to be a "Mud or Snapping Turtle" ?take care as they are fast moving and dangerously aggressive. Once huge enough to eat they make a great meal or soup depending on how you like Snappers. Be careful not to let this turtle bite you and he or she can snap off your fingers, hand, wrist or arm. Once they bite they won't let go of you until the bite is complete. There eggs are larger than other land turtles so they are easy to tell apart.

I hate to burst your bubble, but I doubt it will hatch. Is it a turtle's mating season? It seems weird that there is no other eggs, nest or mama. Maybe if you keep it warm enough, but I think you may hae to turn the egg every few hours, blah blah blah...
It will be very difficult to have it hatch, but I hope it does.
Also, plan ahead. If it does hatch, where will you put it? What to feed it?