Å is in Finnish Keyboards, but totally useless. F12 is easily accessible in Hacker's Keyboard. ∆ is in Gboard.

There no gui, you use openscad to generate STL and view that in Android STL-viewer. You can automatize it so that it is almost like the real thing.

  file_to_watch=$1
  last_modified=$(stat -c %Y "$file_to_watch")
  while true; do
      current_modified=$(stat -c %Y "$file_to_watch")
      if [ "$last_modified" != "$current_modified" ]; then
          openscad $1 -o $1.stl
          last_modified="$current_modified"
      fi
      sleep 1  # Check every second
  done