The official solution would be a
deep knife oven. But, frankly, unless you are also going to heat treat steel, it's a waste of money. You can insulate a box and fill it with light bulbs in the inexpensive ceramic bases from Lowe's or Home Depot will work, as suggested. They won't last long at 300°, but you probably don't need really long life. You can make series pairs of four times as much total lamp wattage to make them last longer.