How do I insulate a neutral bar when installing it in a sub panel?
You should buy an "isolated bar" or an "insulated bar" to install in the box.  This will have insulated mounting hardware of some form (for example, insulated housing for the screw penetrations) which is actually rated for the purpose you intended to put it.

You should probably buy a product from the same manufacturer as the panel you are installing into but here's an example of what you want.