I don't know of any gel pack type of batteries specifically for the 5600. However for my 5600, I went with a rather expensive but versatile cable from Western States Cabling
. I also had my undercarrage battery re-celled at Primecell
. Both have performed well. Western States Cabling is American owned and the products are made in America though the parts may be from overseas. But the quality is satisfactory for me.
I also am concidering the Ebay item mentioned by NYC. Looks pretty decent, but my cable allows direct connect to any motorsport battery. Just have to be careful of the positive and negative leads.
It wouldn't be hard to wire up a plug-in type of connector though....®