I use small boxes I've found at the local 99-cent store. I cut them with the dremel and each gives two one-18650 boxes, to which I then affix the terminals. They have nice plastic spacers inside, so it'd be easy to cut them in anoher shape and have a 2x18650 holder.
Sadly, it appears there are no ready-made holders. You'll have to make them yourself, one way or the other.