Motorycle accessory shops used to sell a clear plastic film for protecting the tank etc.
You can buy in A4 sheets for about £5...it's thin enough to be almost undetectable and thick enough to protect your paint.
I've used it for years.
You mention the shiny thin (self adhesive) stainless steel ones; I have used these too and have about 3 or 4 in my stock - when you find a good product buy a few...that's my motto.
Co-founder of a V-CC section, member South Eastern Road Club and owner of - 2005 Bianchi Reparto Corse (modern), 1985 Raleigh SBDU Pro Super, 1984 Raleigh LU Corsa, 1980 Allin Stan Butler Special, 1978 Ron Cooper, 1975 Bill Philbrook tourer, 2 x very early Roberts (mid/late 1960s), 1966 Raleigh Superbe Roadster, 1964 Allin Stan Butler Special Belgique, 1951 Hobbs of Barbican S/C, 1950 Hobbs of Barbican S/C, 1947 Hetchins Super Special, 1908 Centaur Featherweight. 1988 Specialized Stumpjumper.