I've run across a few ways to keep your downloads secure from others who can just take the whole scene with a nice scraper program.
Here's the best I've found so far (which can be easily done by most any):
1) Put an index file in there - something which sells something for you.
2) In that index file, have it simply redirect to your main sales or squeeze page.
The third isn't actually a way to preserve your downloads - but I've found someone hyping a product which deals with 404 errors, by selling a product which tells people what to do with their 404 errors - meaning put a sales page in there (which sells products about what to do with their 404 errors, by putting a sales page there which...)
But then I found an ebook with resale rights - something like "404 ways to improve your life..."
Best of luck with making your millions.