Mine is similar size, I have plywood that covers most of the top, just set across the top, and a section of hardware cloth on top as well. I use regular construction clamp lights for the birds I have in there now, two 40 watt bulbs but it is in my heated garage that I keep at about 40 degrees. And they stay warm enough (they are a month old) I would suggest getting a thermometer set it under the heat lamp and see how hot it gets (Don't walk away for too long, I melted one that way in just a couple minutes, no chickens in there thankfully) adjust the height of the light and the size of the bulbs to increase or decrease the temp, if a heat lamp bulb is too much go to 100 watt light bulbs and play with wattages and heights. 250 watts might be too much though, the whole box does not need to be 95 degrees, this gives them a chance to move closer or farther away from the heat.
