12v chargers, if they work at all, will just clip all the extra voltage and take what they can use, leaving a max 7a @ 12v. If you get a proper buck constant voltage converter you now have the opportunity for nearly 250w @ 12v = 20a which suddenly makes carrying the behemoth panel somewhat worthwhile. The cheap eBay blue inline chargers will handle your panel at 12v 7a no problem and protect the red arc from the fun and games - just find a naked battery feed like a fridge power outlet or the terminals direct.
Yes, I should have mentioned “cheap”. Anything capable of handling all of that voltage will be exxy and essentially make the red arc redundant like an Enerdrive DCDC but you are talking a lot of money. It would be cheaper to replace the panel as you mentioned and carry on with the red arc in than go that way. At least the little ‘clippers’ or a cheap buck converter will give him a charge at a cost that makes sense for a second hand house panel worth $50 - well that was the point I poorly made in the first place. I agree with you, just trying to get a result that he can live with. I got an ‘indicated’ 9a out of an eBay special pwm on one of those house panels which is a tick in the box for most people.MPPT does not clip voltage, that is PWM that would do that which is only in really cheap solar controllers (or ebay "mppt" controllers)
Either want a panel with lower open circuit voltage or a solar controller rated for higher voltage panels