Single 1 x 18650 injection molded battery sled.
Sold in single units, but two will fit together as in the above images
These work for Alpinetech's 1590A+ (won't fit a regular Hammond A) and fits Modmaker UK's Fatboy enclosure (very similar in size to a 1590A+)
There's also some videos on the official facebook page including some durability tests like running one over with a digger. Yes, a digger.