The diodes get warm, but not hot. The L298N, the larger of the two chips with a metal tab gets warm, but not at all hot. The ATMega168 also gets warm, but the 5V Regulator LM7805, the smaller chip with a metal tab gets hotter than is comfortable to touch in just a few seconds, and it stays hot while the PCB is connected to the power supply, even if it is switched off. Is that normal?
The wooden box is not entirely closed, either. There is an opening about 2" x 4" to accommodate the belt; the PCB is fairly close to that. Maybe I don't need a fan.