Since it's a hang on fuge, just take a few cups of your sand from your display and put in there. If you have some small pieces of rock you can spare, throw those in, too. If you don't have small pieces, see if you can find one medium or large one, take it out of the tank and break it with a hammer, and stick those pieces in the fuge.
Then, all you need is chaeto and your set.
I also have my heater in my HOB fuge...just to tidy up the display a bit.