You need a very clear and sunny day for solar roasting sweet potatoes whole. If unexpected clouds block the sun throughout the day, you'll end up with sweet potatoes that are cooked, but not super soft. Try to have them out in full sun for at least 8 hours. For me, that's 9 AM to 5 PM.
Soak your sweet potatoes in a bowl of water for at least 30 minutes. The soaked water will allow your potatoes to steam nicely throughout the day. This is optional.
Add your sweet potatoes into your black pot along with a small dish of water (optional). The extra water will introduce more steam as they cook so they don't dry out during the day and burn on the bottom. If you don't have a dish small enough, you can add a little water into the bottom of the pot or a soaked cloth or paper towel.
Ideally, you want to be adjusting the solar cooker every 30 - 60 minutes so that it's facing the sun and cooking on high throughout the day.
Bring your sweet potatoes in and let them cool because they'll be too hot to eat. Enjoy!
Notes
If your day was cloudy, you may have to finish them off in the microwave or stove to get them to that soft and fluffy stage.