Your turtle may be more at risk of freezing if housed in a crate in your shed for the winter. It would be a challenge to duplicate the mass (and insulating properties) of the earth surrounding a more natural burrow: you'd need a big crate.
Think of it this way: a box or crate would be surrounded on all sides, including top and bottom, by cold air. While in the ground, a turtle can dig deeper to move away from the cold, but it might not be able to do this if confined to a box.
Also, natural precipitation will maintain adequate humidity.
If your shed is unheated but stays in the 40-45 degree range all winter, it may be an option. I'll post a link to a site that you might find helpful.
Our turtles stay outside year round here in central Virginia, wintering over in deep roomy hibernacula that we created in their enclosure, filled with loose loamy soil mixed with leaves, and covered with even more leaves.
Raccoons have not been a problem for us: they have much more readily available food sources than burrowed turtles.
Stephanie