Two things you can try to cover a thin spot is to brush your hair up in one (hiding the thin crown area very well) and then bun your hair and put a draw string pony tail over it.
This will give you a fuller look and keep the middle section of your hair hidden away.
Another great option for you is a quick weave, which effectively is a cap with the hair glued onto it like a weave would be to your own hair.
The benefit of this is being able to take the "weave" off any time you want and wash your hair underneath. You can also have a couple different ones made so you can switch your style up.
On last thing you can do is start wearing your hair in rod and curly sets and separating the curl to disguise the thin areas by making them look fuller.