The fuse is there to protect the wire from the fuse holder to the receptacle. If you put it toward the end of the receptacle, the wire back to the source is un-protected in the event of a short circuit.

The project box would make a good place to mount the plug though. Be sure that's fastened well somewhere.

Get all new material, cheap and you can get the best quality. A few terminals, 12 gauge wire(go with the same color coding for neg and pos. that the CD has now), a fuse holder and a receptacle(20 bucks maybe?).

I'd skip the solder and use a good crimp on terminal. You can buy one or borrow one or even have someone crimp the terminals and pieces together off the boat to mount yourself.

Good luck, I look forward to hearing about your sailing coming up.