If you're referring to John Leeke's website, I just tried his instructions and made a test version this past week.

My experience is that it takes quite a bit of fiddling with the gasket material around the outside of the frame before it's airtight, given that older windows are never exactly square. I couldn't get one side to fit tightly without throwing the other sides off, and the gasket material kept coming loose and sticking to the window instead of my frame, argh.

I finally shelved the idea for the time being, but would welcome any insights you might have after trying it yourself. It's an excellent idea if you can get the variances just right.

Here is a link that might be useful: instructions for making your own interior storms