Painting the tank black does help a bit, but honestly, it’s not a game-changer once the sun’s gone. I’ve tried both the soft bags and the rigid tanks, and you’re right—the rigid ones hold heat a little better, but they’re still losing it pretty fast after sunset. Reflectors sound clever, but in my experience, unless you’re really committed to building a whole setup, the results aren’t worth the hassle.
Your foam pad trick is probably about as effective as it gets for insulation on the cheap. The only thing I’ve seen make a real difference is bringing the shower bag inside a warm tent or vehicle right after sundown, but that’s not always practical. I wouldn’t overthink it—solar showers are always going to be a bit of a compromise unless you’re willing to haul more gear or get fancy with your setup. Still, respect for trying to squeeze every bit of heat out of the sun... every little bit helps.