Adding a portable panel to roof mounted panel

I suggest the trimetric system. That being said i would NOT put any charge controller in a sealed box. All charge controllers produce heat that needs to escape . The SC3020 with the TM3020 has voltage compensation that would help your situation, well some anyways. Ralph the founder and owner of Bogart engineering lives solar and believes that the battery manufacturers know how there batteries need to be charged correctly and set his up that way. It is VERY adjustable for the advanced user. How many solar charge controllers do you know that can run a bulk charge of 14.6 and when adsorb hits 3%of C20 it clamps the amps and goes into a finish absorb and allows volts rise to say 15.3 then holds it fom 0 to 4 hours? (User adjustable)
Or stop charging at a set amps? (Think AGM type batteries) or overcharge batteries by say 15% as per manufacturer?

And yes i own one........care to guess why? :)