The most convenient space to install a 2nd fuse box is under the passenger side on the finish panel next to the rear heater. We feed this fuse box with a 6ga wire from the 2nd battery in most cases. The 6 ga wire is fused in the engine bay using a 100 amp Mega Fuse.
Typically we use a 12 position fuse box with a 9/3 split, where 9 fuses uses the same feed and 3 fuses a different feed. The 9 is set to be constant power and the 3 are ignition or accessory power.
The location of the fuse box makes it easy to add accessories in the cab.