I really don't like to cross-chain. It's never been a problem before, since I've always had a pretty good sense of what rear cog I'm in. But when I got a bike with a compact double after riding 12 years with a triple, it seems like I'm shifting the front so often that now it's hard to keep track of things. So I made a shift indicator using scrap plastic that's kind of inconspicuous, and you don't need to remove the cable to install it.
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
No, not the toothbrush - the toothbrush package (or whatever other small piece of clear plastic you can find). I used the toothbrush package because the right-angle bend was already formed permanently into it.
And a bit of duct tape, which I forgot to put in the picture.
Step 2: The pointer
Shift into the biggest cog and pick a spot on the cable where you'll be able to see the pointer when you look down. It's going to move away from the fork as you change gears.
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
Cut the window from the toothbrush package.
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.