Is there an aftermarket gas cap gas gauge I can put on my 2003 Honda Shadow American classic edition?

Is there an aftermarket gas cap gas gauge I can put on my 2003 Honda Shadow American classic edition?

Don't know of any.
Best gauge is your eyes, you look in the tank, know your app. Mileage and watch your odometer.

One really cheap gauge is a 16" wooden dowel. You stick into the tank to measure the level and go from there.

Yes.

I'm sure that if you look on the internet long enough you can find one, I did a quick search on ebay and didn't find much.

No.