I dumped it here just now: https://github.com/Noeda/landlock-network-sandboxer-tool

It hopefully will be obvious that nobody should expect quality :) it is like a simplified version of the sandboxer sample in my other comment. E.g. it maybe does not need to touch filesystem stuff at all.

I'd also look at some of the sibling comments for maybe more refined tooling than this thing. Maybe it's useful as a sample though.