This works not really well, i3 will return "multi document" json and main split will be lost.

i3-layout-saver.sh

#!/bin/sh

DMENU="rofi -p SaveLayout: -bg #922713 -dmenu"
if ! command -v "jshon" >/dev/null 2>&1; then
    printf "%s\\n" "you need to install 'jshon' to run this program" >&2
    exit 1
fi

if ! command -v "rofi" >/dev/null 2>&1; then
    printf "%s\\n" "you need to install 'rofi' to run this program" >&2
    exit 1
fi

LNAME="$(ls ~/.i3/layouts | ${DMENU})"

if [ -z "$LNAME" ]; then
  echo 1
  exit 1
else
  echo $LNAME

echo '// vim:ts=4:sw=4:et
{
    // this is a manually added container to restore the vertical split
    "layout": "splith",
    "percent": 0.5,
    "type": "con",
    "nodes": [

' > ~/.i3/layouts/${LNAME} 

  WORKSPACE="$(i3-msg -t get_workspaces | jshon -a -e focused -u -p -e name -u | awk 'f{print;f=0} /true/{f=1}')"

  ## Last sed is for some few problems with save-tree returning "} {" without 
  ## comma, i dunno regexp but "multi document" json wont fix itself.
  i3-save-tree --workspace ${WORKSPACE} | sed 's/\/\/[ ]"class":[ ]"\(.*\)",/"class": "\1"/g' | sed ':a;N;$!ba;s/}\n\n{/},\n{/g' >> ~/.i3/layouts/${LNAME} 

  echo '
    ]
}
' >> ~/.i3/layouts/${LNAME} 
fi