I got a combination buck / boost DC-DC converter for $13 shipped on ebay.

It might need a cooling fan, we'll see. I will use it to convert the 15-20V from the panel to 12V to charge a standard 12V battery, or to charge a 18V or 19V laptop (there will be a way to switch between the two modes and connect the battery or the laptop. If the output is not stable and goes over 18 or 19 V, I will add an 18 - 20 V adjustable regulator circuit to the output to protect the laptop. When the main converter is in 18-19V mode, I will presumably be using it with a laptop, so I can charge USB devices from the powered ports.

When charging a 12V battery, I'll use a $4 12V-to-5V module to charge USB devices from the battery.

I'll also put $2 LED voltmeters on everything.