actually, there may be an easier method. however, not without a little wiring know how. and you definitely don't want to exceed a charge rate higher than the 12V socket was intended for. there are other threads on how to make a direct 12V connection from the fuse box so i won't post it here.
you would need to have a direct always-on 12V connection (either from the 12V battery terminals, or fused connection point).
i use a goal zero sherpa 50 to power a dashcam for longer periods when i'm out of town. it takes a 12VDC from the 12V battery (via the fuse box), and outputs 12VDC to the dashcam. this connection is always on, and i have a box that monitors the voltage which will shut off the connection if voltage drops below 11.6-11.8V to keep the car's battery from going flat.
these units were designed with using solar panels for on-the-go power (and off the grid) and they have a series of solar panels that work with a variety of lithium-ion power banks/generators. so all you would do is set the 12VDC (or whatever output) from the solar panels, and output the 12V from the sherpa (or similar charger/powerbank) to your 12V socket. basically reverse of what i use mine for.
a bit pricey for similarly performing chargers on the market, but i found them to be pretty robust and reliable. i have had one connected to my car may be a year or more. they are very modular so i can even take it out of my car and power a host of devices if i'm at the beach, camping, etc.
here's the one i use (it outputs max 8A on the 12VDC out)
Sherpa 50 Power Pack | Power Packs | Goal Zero
and you would need this cable (or similar)
Guardian 12V Adapter | | Goal Zero
as with any modifications and add ons, i highly recommend monitoring consistently and make sure you have good connections. check for any codes thrown from the car and always use a fuse. don't attempt if you're not comfortable with wiring.