The soda water bottle pasteurizer shown here may have better performance than the one shown at Soda bottle pasteurizer. It may also be slightly simpler to fabricate.
The cut off base, inverted, provides a stand for the blackened pop can (or other suitable black container). The taper of the inverted base eliminates the need for tabs--the cut off top just jams down over the inverted base.
Performance might be better for three reasons. First, the stand raises the pop can so that its bottom can be illuminated. (Be sure to blacken the bottom of the pop can.) Second, raising the pop can improves its bottom insulation. Third, the air leaks are below the bottom of the pop can. Since all of the air in the bottle above the stand is hotter and lighter than outside air, this stable arrangement of cool outside and hot inside air resists air leaks. In the design of Soda bottle pasteurizer, the air leaks are above the pop can, promoting outward leakage of hot air and inward leakage of cool air.
A Possible Further Improvement
As the water in the pop can gets hot, most of the heat loss will occur from condensation of water vapor on the upper inner sloping sides of the soda bottle. We should be able to reduce the rate of heat loss from this condensation by double glazing the upper part of the soda bottle using the top portion of a second soda bottle as a hat on the first soda bottle.
The hat should accelerate the final portion of the temperature rise, or allow reaching a higher temperature (to make tea?), or allow successful pasteurization on less sunny days.