How to override "Application deployment in progress" page?

[djangodude Profile]

On http://www.helicontech.com/zoo/docs/deployment.html it mentions:

If you want to replace this page with something that fits your web site design you can simply override 503 error pages in the IIS settings.

Could someone explain in detail how this is done? I tried creating a simple 503.html page and set my app to use that (Sites -> My Site -> Error Pages, add status code 503, select "Execute URL on this site", enter URL of my 503.html). This causes an error ("unhandled exception in w3wp.exe [7908]"). I also tried using the "Insert content from static file into the error response", but that caused a 500 error.

So are there any detailed instructions on how to properly override Zoo's default 503 with one's own custom version?