Integrating Market Theme with e-junkie.com could be a good solution for that.  E-junkie.com is about $5 per month, but they give the ability to handle digital downloads with good security features.  The download page is dynamic and changes each time, and can be limited to only be available for a certain amount of time.  (ie. 3 days or so)

You could also do the same thing using only Market, however the "download" page would be a static page, that you may want to rename periodically.  (Eventually, you'll get people sharing the download page if you don't.)