I use the fuse block that Twisted Throttle sells. It is positive (red) only.
So I have to provide negative (black) to all the devices powered off this fuse block. Which is easier than you think.
It also has a wire pair to which you can splice your Battery Tender pigtail to. The custom relay that comes with the block knows when you plug your charger in and keeps the charging system seperate from the other devices powered from the block. Some electronics can't take more than 12V DC very well, and a charger will often go over that.
Also, if you turn the key on, the fuse block relay isolates the charger circuit from the bike totally (turns the charger off, essentially) for the same reason. Turn the key back off, and the charger comes back on.
It doesn't have a nice cover like the Fuzeblock but I imagine if I do another one for my C14, I can machine up a nice box for it to live inside. Heh.