Mike:

I did the same thing on my boat: wired four solar panels totalling 220 watts
into a junction box on deck and then one 10 gauge cable to the DC panel.
West Marine and others sells a junction box with a phenolic base and cover.
It is about 4" x 6" x 2" deep and has a terminal strip with just enough
screw positions for 4 conductors in and 1 out, pos and neg. The cover has an
o-ring seal and there are inlet and exit grommets for the wiring. I had to
do some grinding of the inside of the cover to get enough clearance for all
of the wire and terminals. But it worked.

But you almost certainly can't fit 6 gauge wire into the box. But 10 gauge
should be sufficient. 300 watts of solar panels should put out 20 amps while
charging at 15 volts at the panels. With 10 gauge wire and say 20 feet each
way to the DC panel or wherever then the voltage drop will be only 1 volt,
giving you 14 volts at the other end. This should be enough to charge your
batteries at 20 amps.

And unless you have a mount for your panels that tracks the sun, you will
only put out the full 300 watts for a few hours each day when the sun is
near 90 degrees incidence to the panel face. FWIW a real world average is
that your 300 watts of panel capacity will put out 100 amphours of energy on
a full sunny day. So, the average current will be much less than the 20 amps
at peak.

And finally be sure to use a solar charging controller and a 30 amp fuse or
circuit breaker on the battery end of the circuit.

David