There appear to be several methods for enabling any app to work with multiwindow mode if you have a rooted device but I did not want to root mine. I found an app called MW-Pen App Enabler Pro which does the job for non-rooted devices. It will let you choose your installed app, modify it on your phone, prompt you to uninstall and reinstall using the app itself and the end result will be a multiwindow compatible app. The process was pretty quick even for fairly large apps. There are manual methods described on the web that involve decompiling, editing and repacking an apk but this app will take that effort out and do the job smoothly.
The app is free to install on the Play store but in order to start modifying apps you need to purchase the Pro key which is a one off cost of £1.50. I modified TomTom and Audible which immediately appeared in the multiwindow drawer without even a reboot.
I have no stake in the app but it has made my life a lot easier and I congratulate the developer on his great work.