Just a heads up for next time, it helps everyone whom you are asking for help if you post some reference pictures. Not only that, but it shows you are invested enough in the project to be doing your own research, which makes us more compelled to help out. Not trying to sound preachy, just something that may help you get some more responses in the future.
Based on what I looked up, I'd say your two options are to order a cheap pair of knee high costume boots with a zipper in the back and customize the outside with craft foam, or craft the entire boot extension out of thicker EVA foam and build them up from an old pair of shoes. Those would both be relatively quick ways to go about this that wouldn't cost an arm and a leg.