>>1648870Any matte/flat black paint will do. You don't need any reflection inside the tunnel, but having the reflector outside, so more light shines into the tunnel onto the drum, will help. You can see this type of thing being used for solar box cookers. The black object is inside the glass window and insulated box, but its reflector is outside.
In your image, the reflector needs to be outside and behind the drum in relation to the sunlight. That way the structures don't shade your plants or other parts of the tunnel.
While Mylar is optically optimal for reflection, flat/matte white paint is 2nd best, easier to work with and will last essentially forever. The plastic the Mylar is made from will degrade over time. The difference between Mylar and white paint isn't enough to warrant using Mylar.
>>1648880>bury the bottom third of the barrel in your dirt.The extra thermal mass is more about maintaining warm air temps than it is warm soil temps, fyi. Also, added reflectors outside that strike on the barrel will be more efficient than added barrels as far as space goes.
