I undertook this fuse box upgrade this weekend. I had ordered the fuse holders back when @Phil Z first posted this. Just now was motivated to do it. My fuse box had the typical melting of the parking light fuse. I installed an inline fuse pre-1980, and installed LEDs a few years ago to lower the current draw. My first step was to take photos of the existing wiring and also make a key to where each wire goes. Next I removed the fuse clips leaving a bare box, then repaired the hole from the melted fuse with JB Weld. Semi-ground down the JB Weld and coated it with black satin paint.
Like Phil I used a dremel to cut slots for the fuse holders. I cut the first one too wide, so after that cut them slightly smaller than needed and filed them to the right width. I had some 6/32-inch machine screws in my stash, some 3/8" long and some 1/2" long, and they were the correct diameter. Drilled out the holes as needed and installed the fuse holders and screws.
Nissan didn't leave any spare wire length, so I pried open the clamp holding the insulated part of the wire and cut off the old fuse holders to give me all the length I could get. To connect the source side of the fuse block where two wires were common, I tore apart an old plug and soldered those across the terminals. At the three common connectors the stock box has a screw terminal. To duplicate that I smashed an old duplex outlet and soldered the parts across the three terminals. Ugly but effective. I soldered the wires on, checked for continuity, and re-installed it in the car. So far everything works like it did before. As Phil said, the fuse box cover still fits.
Thanks to Phil for coming up with this upgrade.