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I covered the exterior of mine with metal tape (for use in HVAC applications) prior to re-install. It didn't make it any easier to install per se, but it should keep the somewhat flimsy unit from deteriorating prematurely...
Take the door off, but also the latching mechanism on the top inside of the opening . Its held by 2 screws and pushes off with a little force. Then you can (massage) the box in easily, just dont force it. Cardboard is flexible, just me careful.
Check out the FTE Tech Articles. There's a new article on there, that might provide you with what you're looking for. When I get the free time, I make them and put them up on Ebay. If ya need help or have questions about the process then lemme know. If any of you local people need help lemme know. I've got all the metal working tools in my garage to make these.