Are you keeping your turtle outside or in? More details on how you are keeping the turtle you have would be helpful.
All turtles, and especially ornates, do better when kept in an outdoor pen. If your turtle(s) are being kept in an outdoor pen that has dirt soft enough for digging, you shouldn't need any kind of a nest box. I'm not even sure a turtle would willingly nest in some sort of box. I have one female (michelle) who laid her first clutch (with me anyway) in her pen, but each year since, she has paced the perimeter of the pen until I took her to a spot in the backyard where she would find a spot to nest. My theory was that my pen had become increasingly shaded and the area she nested the first time was not getting much sun. Last night, after watching her pace for several nights, I took her and another pacing female(Juniper) to the area along our back fence in the back yard. Juniper found a spot and started digging right away, Michelle paced up and down the fence line for an hour, so I gave up and put her in the pen. When I went to check on her an hour later, she was digging in some very soft dirt in the pen. When I went to dig them up this morning, it was a good thing I did, because she had laid 5 eggs,in a pretty shallow nest. Had I stepped on that area, they would have been crushed, or another digging turtle could have easily bumped into them. My point here is this isn't an exact science and you can't force them to nest in one particular spot.
Also, I wouldn't recommend getting a breeding "group" until you've been through the process at least once. Consider finding one "mate" for your turtle and see how that goes.
