Since first asking the question I have found a company who make and sell (worldwide) something called a “Magnetic Door Defender”. They are (in my opinion) less appealing than the suction versions above (which I cannot find in the UK), but you can buy them in pairs online. The original USA company website is http://www.doordefender.com and someone selling them in the UK can be found at Cushioned and Magnetic car door protection system - More Than Polish. They stick to the door magnetically and have a security cable that you lock inside. That said, I have just bought some foam pipe insulation tubes (3 inch diameter) and I intend to follow the instructions and make my own.