I get why the pithy "chroot with more steps" comment has been largely dismissed, but when all you're going for is the temporary installation of build dependencies without impacting the permanent system, chroot really can do enough. The other stuff with user namespaces to isolate process IDs, networks, and map UIDs to get root in the user namespace without actually being root, aren't strictly needed.
There is an Arch Linux devtools project providing some shell scripts to automate setting up clean build chroots, but they're of course focused on setting up an isolated Arch system, and still require running as root. The true poor man's way to do this without being root is to use fakeroot and fakechroot, which is what the Debian builds do. The examples from Debian would all run debootstrap to set up a minimal Debian in the chroot, but you can just run "fakeroot fakechroot <any command>" to use your own build tools and bootstrap your own build environment. That way you don't even require a container runtime and don't need to open up the attack surface of user namespaces.
Along the same likes as fakeroot/fakechroot, I've found proot to be super useful for writing startup scripts for commands which make awkward assumptions about the filesystem layout.
There is an Arch Linux devtools project providing some shell scripts to automate setting up clean build chroots, but they're of course focused on setting up an isolated Arch system, and still require running as root. The true poor man's way to do this without being root is to use fakeroot and fakechroot, which is what the Debian builds do. The examples from Debian would all run debootstrap to set up a minimal Debian in the chroot, but you can just run "fakeroot fakechroot <any command>" to use your own build tools and bootstrap your own build environment. That way you don't even require a container runtime and don't need to open up the attack surface of user namespaces.