I think most people actualy wire direct to the battery with both wires and use at least 8 gauge (ie don't use 10 or above).
the fuse you need is going to be about 15A and should be as close to the battery on the +12V line.
There is a thread here with some info:
Power wires inquiry
Edit: to do a simple fuse calc: power/voltage=current so 150/12=12.5A. If you find you ar eblowing fuses when everything is running (the opus can produce over 150W i believe and there are also bad connections etc that use current) then you may want to replace it with a 20A or something in between if you can get it.