Warning - if you get frustrated fiddling around with small things until they're just right, don't even start this project. And, I'm the only one who ever handles my bike, so the fact that it's a bit fragile isn't a problem for me.
Step 1: What you'll need
And a bit of duct tape, which I forgot to put in the picture.
Step 2: The pointer
You need something that will attach securely enough to the cable, and be visible enough to see through the window. A piece of duct tape works well. Cut it about 1/4" wide and 1-1/2" long and wrap it tightly around the cable. Tweezers are helpful.
Step 3: The window
The window needs to be about 1-1/2" long, and wide enough to go out from the frame about 1/8" past the cable.
Stick it onto the down tube with double stick tape, referring to the pointer to position it.