Thanks for the tips! I think I'm going to go with Larry's suggestion
and build the one he posted. The 110V adapter that came with the unit
states that it has a 2.4 amp output, but I'm sure that's only required
during charging. Whenever I use this player on the boat it will already
be charged. I'm still going to add the heat sink right off the bat
though.

I want to stay away from using an inverter, and the only DC to DC
adapters I could find pre-made didn't have a 5V position, but thanks for
all of your suggestions anyway.