A temp of 104.3 under the lamp is okay, as long as the brooder is big enough that all the chicks can get out into room temperature air as well. If you can raise the lamp, it will be cooler underneath.
The brooder should never be a closed in box, it needs plenty of ventilation to prevent overheating as chicks do give off body heat (especially broilers), and to vent moisture and ammonia from urine. When brooding indoors for the first two weeks, I use a 95 watt bulb the first week, a 67 watt bulb the second week, then I move them to the coop under a heat lamp suspended from the ceiling. By that time they have learned how to regulate their body heat.