You can create a simple form and if the user submits he gets redirect to the thank you page with the download link.
Another option is, to send the user the download link via the entered mail. I think that will save you more from trash mails.
Since the Craft install is hosted at Hyperlane, they have a way of locking changes so that you have to push to the repo through git and deploy. No changes can be made to the filesystem through the admin UI.