Where in "I have a problem holding thin aluminum, about .080", together for a period of 3 - 4 months in an outdoor environment." did it say it needed to come apart? or how often? He just said it needed to stay together for 3 to 4 months - which his method was not doing. If he gave us an idea of the application we could give him an intelligent solution. He says he only needs 4-5 lbs force but does not tell us what shear strength he needs. He did not say how often it needs to come apart and go together either. Mabee a latch, like a Dzuz is required. Mabee a riveted on piano hinge with a removeable wire would be the proper solution. Mabee it should have a thick bar fastened to the back of the back sheet, threaded for bolts or machine screws. If the holes are near an edge, mabee he should use tinnermans. Mabee he should rivet on nut-plates.
If it only needs to stay together for up to 4 months, once, nothing wrong with pop-rivets - either use backing washers, or just use enough that they hold - do like Zenith Aircraft does - use 50% more than you might possibly need for the calculated forces. (he uses avdels - which are really just "super pop-rivets" and not NEARLY as expensive as Cherries)
Different horses for different courses - and we have NO idea what his application or requirements really are.
(and he DID say he "taped" the screws into the metal - what kind of tape did he use - mabee better tape would do the job - I ASUME he meant he TAPPED them. - but that's not what he said - - - - )