You don't really need much circulation. The fact that the air inside is warmer than the air outside the enclosure will promote plenty of circulation as the warm air vacates the top of the enclosure and cool air replaces it. My 3x2x1 PVC enclosure only has two slots about 3 inches long and 1/4" thick on the top back sides, plus whatever air gets past the "seal" of the acrylic hinged window in the front. Plus, it's not as if snakes breathe large volumes of air, they have itty bitty little lungs compared to us. A few square inches should be more than enough circulation.