I have used my self-made slatts belt for over 2 years.
It is certainly stretchy. There was some distortion in the first few weeks, but it was fairly uniform and stabilized out. The stretchy is nice from a comfort standpoint, poor from a load bearing standpoint. Now that I recently switched to a double layer leather gunbelt, I notice my pants no longer droop down in back, and also that I feel belt pressure after meal time.
The slatts belt made from paracord is thicker than a leather gunbelt. I could not fit many belt accessories like pouches or holsters on it.
Overall it was comfortable, and worked about the same as any other soft, stretchy belt.
Unless you want a project, there are several internet craftspeople that make them for a reasonable amount, vs the value of time spent crafting one yourself for the first time. Look to places like Etsy, Ebay, forums, etc.