Paul, if you have a UPS Store near you, they sell boxes that are perfect for shipping a rifle. They call them a "golf club" box. Measures 6x6x48" and costs about $4.00. They also sell bulk packing peanuts. Bubble wrap the rifle and fill the rest of the container with packing peanuts and it's dang near bullet-proof. They also don't scream "gun".
Great thing you're doing and some kid is going to be thrilled!
