Welcome to the forum! Interesting experiment with the innards of a solar heater. A solar collector box as small as a 2'x4' box might not give you definitive answers as to comparative performance, it should be able to quickly and inexpensively eliminate any non-contenders. As far as painting the inside of a box, remember that the back of the collector box also has a role to play in the overall performance of a collector. It is true that a collector absorber that prevents any sunlight from reaching the back of the collector. But any sun that hits the back of the collector through layers of screen also hits the screens a 2nd time. So the sun entering a 3 screen collector actually has to pass through 6 layers of screen before exiting back through the glazing. Or as per your example, 50% of the sunlight gets through the 3 screens on the way to the back surface of the collector. But that also means that 50% will be absorbed that reflected off the back meaning virtually 100% of the sun’s rays entering the collector box are absorbed by the screen mesh.
But while you are trying to compare the difference in the performance of the various absorbers inside a collector you may want to consider painting the inside of all of the collector test boxes. The reason being is that some collectors use the heat absorbed by the black paint of the back of the collector more than others. For example, a three screen absorber can catch the suns rays both entering and exit the collector box. That’s effectively 6 layers of screen for the suns’ rays to be absorbed by the screen mesh. Not much solar energy will be lost after having to pass through 6 layers. Whereas an absorber made up from mini blind slats effective prevents virtually any solar energy to hit the back of the collector box. Yes, heating the actual collector box is not the goal, heating the absorber is. But when it is frigid on a winter morning, such a cold box will not run properly or at all until the interior of the solar heater box warms up. My suggestion would be to consider making a third test box, painted flat black, with no absorber of any type. This can be your control unit when testing various absorbers.
A 2x4 passive collector box is a not lot of space for proper testing, especially being passive. But it is inexpensive, easy to build test unit. Once you have narrowed down possible top performing absorbers, you might consider building larger test units to give a more accurate indication of top performers as small test units can have very small, subtle differences in test temperatures that even slight variances during the build may enhance or negate changes to the absorber.
Again, welcome! And thank you for experimenting and sharing. The discovery, or chance of discovery is what makes this exciting!
Greg in MN
PSlease forgive the rambling as I was quite distracted while writing and I was too tired to do any editing before posting.