You want to use a large wire for this jumper, so you can do whatever you want in the future. Now since its only fishfinders and you could pull maybe 10-15A i would say 10awg is enough. But i would personally use 8awg for this, if not 6awg.

If you dont want to use a fuse box you dont have to, i personally like solid bus bars for boat installation. Then i run a fuse on the end of each individual positive wire. Same thing though really, i just dont trust cheap fuse boxes and good ones are very expensive.

If you have room, you can also use the bolt on bus bars for lithium batteries, they screw right into the terminals and allow you to attach multiple items directly to the battery. This is the best way if you have the room in your battery box, and put individual fuses on each wire.