Narva sell a six way Blade fuse box (Part. No. 54422BL) that fits neatly in front of the GL1800 fuse box.
The box costs about $14.00 and you will need some cable ties, a 30 amp relay and cable etc.
This is an individual fuse box but you can loom one side or bend and solder the lugs together. Run heavy cable, at least 30/0.20 from the battery to the switch side of the relay. Run smaller cable from the accessory outlet on the GL1800 fuse box to the coil side of the relay. Then run the respective size cable to the fuse box and back to the -ve side of the fuse box.
Take a look at the picture below for the idea.
Cable tie the fuse box in position and be sure that you either insulate or physically keep the loomed terminals form the frame, (I use 3M splicing tape for these sorts of applications. It works brilliantly).
The circuit diagram is below.