you can use a long hex coupling nut, with a bolt to have an adjustable spacer.... makes it fool proof, if you ever get other sized
batteries...
or if you want to dedicate yourself to a size, you can always take two copper pipe caps and affix them together with a short piece of copper tubing. I've seen it used in tube
mods (with heat shrink, to prevent shorts.
wrapping the sides up in electrical tape insulates the spacers from foreign objects while charging.
Im not a fan of having that much metal exposed while charging on a dual charger. resting a tool or walking by with a tool in our hand can lead to an awakening shock... or having a stupid dog lick it
thats a possible 8.4V you can be touching. even though its no enough to kill you, it can be enough that a short can cause failure of charger or
batteries, and we all know that is always a bad thing.