before you paint it black can I ask why?
I am not trying to be mean or pick on your but you plan to travel into a tropical area where both white and reflective are great things.
I really really wouldn't paint that shell black.
I would like to hear more about your route, skill sets, who all is going and such. It might be that many of the folks who have done this trip before can be of help.
You might want to look for a book called Americas Overland by the Greenes (they post here). It covers Central America well.
I would also recommend the Overland Training Comp Course before you invest too much in gear, rig or stuff...the price might seem high at first but when you make it a percentage of your total trip budget it is cheap and when you think about all the mistakes you could make in border crossings, gear selection or driving then it is priceless.
9 months seems like a long time but I know from all my experience that the time flys by and you are always staying up late at the end to make things happen.
cigar smoking, wilderness first responding, ham talking night nurse who is overland certified and a fellow of the Royal Geographical Society.....
now everyone say "so what where have you been lately?"