Get an ABC or two (you shouldn't need a Class D) and mount them somewhere out of direct sunlight.
I find that putting a smallish (2.5 - 5#) unit in a clamp on the floorboard between the driver's seat is usually both convenient (you can grab it in a hurry) and out of directl lighting (so it's not hot when you grab it.)
You can put a 5-10# unit somewhere in the back in a corner, and put a towl or a light cloth over it (like a section of bedsheet) to keep direct sunlight off of it. Granted, they're overbuilt for a reason (you'd be surprised what those cannisters can take!) but why buy trouble when you get so much for free?
You can put it in the sunlight for a while without trouble, but there's no reason to not give it a little help. Make the towel, sheet, or whatever have a light colour - to better help reflect infrared (white is best, greys and earthtones work well - AVOID RED, PURPLE, BROWN, and BLUE! All are made with red dyes, and they'll actually absorb infrared, rather than help to reflect it...)
5-90