I’m having trouble following what you think your site organization is, and why domain names would be relevant. If I’m completely off-base below, you’ll want to clarify what you’re trying to achieve.
[Remember that github only serves static sites: if you want to show off sites which are dynamic, you’re going to need to host elsewhere.]
This is how I’d do it:
I’m going to assume that you have a domain, “DePaepe.org”, which is your “portfolio” / “hire me” site. This could of course be “DePaepe.github.io”, i.e. your github page. This site then has links to your “demo sites”.
Each “demo site” would be the pages for that site. Each site is in your github repo as separate folder sub-trees. So your “portfolio” page has a text label that says (e.g.) “FancyShoppingSite.com” but it actually is a link to “DePaepe.github.io/FancyShoppingDemo.html” [or whatever the actual syntax would be].
As long as each demo site files only use relative links/references (i.e. doesn’t have a reference to “FancyShoppingSite.com” someplace), I think this should work.