I'm not personally familiar with the layout of making one, I imagine if you knew how to tie slip knots you would definitely be able to make it without the tubing, AFAIK all it does is allow you to adjust the length of the component loops to fit your body tightly.
If you use 550 cord you'll want to inspect the harness occasionally as it is quite prone to damage by abrasion with itself (you can cut paracord by abrading it with another length in seconds).
You may be able to use a more durable cord if you do some research.