Another vote for the hiking map carriers. Ortlieb, I found to be the best.
I used one for years, with no problem at all on assorted motorbikes, clocking up tens of thousands of miles. Eventually, my original carrier turned dark brown, I guess due to a mixture of UV and accumulated grime, so I bought a new one.
Easy to use and reversible.
A friend has made his own for his Triumph Explorer. He bought a second hand U-shaped piece that screws to the filler cap ring. This was made by the bods who make the tank bags, I forget the make. He also bought its female accompanying part.
He then hunted around for a suitable map carrier case, finding on that was good enough but was too flexible. To take the flex out he slid in a thin plastic sheet, rigid enough to support the carrier. He then carefully bolted the female accompanying part to the rigid plastic sheet, mimicking the way the sister part mates with a tank bag.
The whole then mounts, via the U-shaped pieces, to his tank. To all intents and purposes it looks (and works) as if he had bought the whole lot direct from a manufacturer. You would never guess that it was homemade. It has now done over 30,000 miles with no problems at all.