Bash, Fish: prevent variable expansion

Am planing to switch to fish as my default shell but am having some troubles with it.
Before rewrite all my script I would like to reuse the old ones using bash with '-c' option.
Example:

bash -c "cat <<'EOF' | tee filename >/dev/null$SHELL$TERMEOF"

Even using 'EOF', I always get:
/bin/fish
xterm

If posible, I would like to avoid to use a backslash \$SHELL in every variable in the script.