If you’re not stuck with the 12V panel, you can use a 6V panel and large Li-Po, with no other hardware required for the Argon.
But since the Argon is a hungry device, sticking with a 12V System is a more robust solution for extended cloudy days verses the cost of a very large Li-Po.
This is a general 12V Solar & SLA Combo that I’ve had good luck with in the past for less than $60 USD.
10W Solar Panel, 12V, with charge Controller
Any 12V SLA battery
Any 12V to 5V micro USB
The only physical connection to the Argon is the USB, of course.
If you want the Argon to monitor the 12V battery, you can also add a 5:1 voltage divider like this and sample the 12V SLA battery with an Analog Pin.
Another Add-On is a 12V Timer Relay since you have much more capacity with the SLA than you need, wasting a little doesn’t hurt a thing, $5 timer relay . This will minimize the chance that you will need to travel to the site to Power Cycle the Argon (Borons, in my case) in the future. I use the timer relay as an adjustable external WatchDog and the Particle Device will subscribe to it’s own scheduled WDT Publish Event. If the WDT publish doesn’t make it to the Cloud and Back, the Timer Relay will perform a complete Particle Power-Down with a defined OFF interval. I use 15 seconds without power when something goes wrong with the Cloud Connection. The Timer Relay kills the 12V power path to the 12V:5V USB converter when it times-out from missing 1 (or multiple, you decide) scheduled WDT subscribed events.
This inclusive combo above is a pretty robust solution, if physical size isn’t a concern.
I know this isn’t a direct response to the original question/post, but it might be an alternative worth considering?