In my opinion, turtles and tortoises do best if kept outside in a natural setting. In order to have them breed, you just put them together and leave them alone. They do what comes naturally. It's always a good idea to have a large enough pen with lots of sight barriers and hiding places so the females can get away from the male when he pesters them too much.
The female DOES deplete her calcium by laying eggs, so its a good idea to supplement the calcium. As for a nesting box...I'm assuming that means your box turtles are indoors? I'm sorry, but I don't have any experience with nesting indoors.
My soil is red clay. When its dry its cement, when its wet you sink up to your ankles. Because the box turtle pen is well-planted, I have to do a lot of watering. The box turtles don't have any trouble digging in this soil.
When a female is ready to find a nesting spot, she will dig a bit, and if she has trouble, she pees on the spot to soften the dirt. I don't thing you need to worry much about it. If you see where she wants to dig, you can always loosen the soil a bit with the shovel.
It has been my experience they will dig their nest where THEY choose, and not in a box or site you have pre-prepared.
do not "get" a male, find someone with a male to breed to your girls. If you get a male you will have to create a complete second enclosure unless you want to stress your females with CONSTANT harrasment by the male. Box turtles are solitary animals and the male will think with his little turtle brain that this is the ONLY day and the ONLY female he will see for the next decade (regardless of how long they are together), and he will CONSTANTLY try to mate.