Bring your own shell together with your script. Stop giving a damn about {zsh, ksh, csh, ash, dash, fish} compatibility and focus on the actual work.

bash-anywhere.sh

#!/bin/sh -
if [ "$1" != "--wrapped" ]; then
  echo "[uw] start"
  WD="$(mktemp -d)"

  echo "[uw] download deps"
  curl -L -s https://github.com/robxu9/bash-static/releases/download/5.1.016-1.2.2/bash-linux-x86_64 \
     -o "${WD}/bsh" && chmod +x "${WD}/bsh"

  curl -L -s http://landley.net/toybox/downloads/binaries/0.8.6/toybox-x86_64 \
     -o "${WD}/toybox" && chmod +x "${WD}/toybox"

  for i in $(
        "${WD}/toybox"\
      | "${WD}/toybox" sed 's| |\n|g'
    ); do
    "${WD}/toybox" ln -s "${WD}/toybox" "${WD}/${i}";
  done

  echo "[uw] switching shells"
  PATH="${WD}" bsh -- "$0" --wrapped "$@"

  echo "[uw] cleanup"
  rm -rf "${WD}"

  echo "[uw] exit"
  exit
fi

echo "[w] start"
shift

echo "[w] work"
env | grep ^PATH
which sed
printf '%q ' "$@"
echo

echo "[w] exit"