An under tank heater would be much better for heating than a light, the light will dry the cage out too much, as will the screen top. They also need belly heat to digest. Sand is not a good substrate for a BP, newspaper, aspen, or cypress would be much better.
Congrats on your first BP. I can't really help with the sexing much.
I'm sure others will chime in.
I would use foil to cover most of the top so that the humidity stays in.