WP plugin/script to handle link redirection for CPA offers VS. making 20k separate pages??

I'm working on a wordpress site based around offering thousands of downloadable files that are hosted externally (Rapidgator).

Here's what I'm trying to do:

1. User visits post on site and clicks link to download the file of their choice.

2. User is redirected to some kind of interstitial or in-between page where the script that loads my CPA offer is executed. The script will effectively lock the page so the user can't continue to the download link unless they complete the offer.

3. User completes the offer and is either automatically redirected to the download page (ideal), or they are redirected back to the interstitial page where they can click a link that takes them to the download page.

Any ideas??

I could probably find a way to automate the creation of thousands of standalone redirection pages for each post/file on my site, but that just seems silly and like a massive pain in the ass to keep track of.

Is there a WordPress plug-in or some kind of .htaccess rule that could achieve this with just a single page/template instead?

I know creating a redirect to a page isn't that difficult, but I'm not sure of how to pass the URL of each downloadable file to that page for the user to receive after they complete my CPA offer.

Any advice or insight is greatly appreciated.