Run Chrome with file access AND in a separate instance

Sometimes you want to run your browser without some protections that are enabled by default in it. One of these is the cross-origins protection when running a local filesystem script. For Chrome, the solution is to run the browser with the command line switch --allow-file-access-from-files. Seems straight forward enough, but if your browser is already open (usually I have at least 10 active tabs, ranging from documentation pages, email to the music I listen to), the command line switches will be ignored and your script will be run as just another window in the same instance of Chrome. In order to fix this, you need to use another switch called --user-data-dir. Just make sure this folder exists and it can be deleted (because it will be filled with a zillion useless files).

So, how do I run a test.html file that can access files and is located in C:\SomePath ? Like this:
start "Chrome with access to files" chrome --allow-file-access-from-files "file:///C:\SomePath\test.html" --user-data-dir="C:\SomePath\UserDir"

In your path a UserDir folder will be created which you can delete after you finish your work.

Of course, this issue applies to any other flags that you want to use ad-hoc while Chrome is already open.