Have you used the Velox brand tape in a width that runs the width of the bottom flat of the rim? They make a wide range of widths. I've even taken a wider width and cut the roll with a very sharp kitchen knife (holding the roll in a bench vise) to make a custom width (only because I needed a roll, what I had on hand was either too wide or too narrow and this is COVID; I go out once a week).
The Velox tape is cotton (I presume) but works very well. From a consortium of French companies that have been making bike stuff for a century. Clean and dry your rim first. The tape should hold up for years, even decades.
Ben