The fuse by the battery is there only to protect the wiring -
ie if you short the end of the wire back in the trunk to ground, or
some piece of metal chews its way through the insulation in a couple
of moths, it will make sure that you don't melt the cable and set
everything near it on fire.

So the exact rating isn't all that critical, as long as it isn't too
low (triggered by normal playing) or to high (allows cable to melt
without triggering, which is the worst case...).

One idea: sum up the fuses on your amps, then apply reality factor
(you can normally go somewhat lower than the sum). If you think you
might add more amps you mightas well use the bigger fuse at once
as long as the wire can take it!
I'm sure someone has a table of max current capacity for different
gages of wire. OK, hang on... This might be useful...

If you branch out from a distribution block, don't forget to fuse
there too, according to that wires (probably thinner) capacity.

Mounting varies with brand/type of fuse holder, but normally the wire
attachment is sort of compression fitting, and the holder has a small
foot by which it can be mounted with screws or double sided tape.

/Michael