I second the idea of building your own deb.
But if you're going to learn a DSL for building debs, why not just learn how to build debs? Your distro already provides the necessary build formula, along with well-integrated startup scripts, etc.
To start out, just build as given:
apt-get build-dep nginx
apt-get source nginx
cd nginx-$VERSION
dpkg-buildpackage
Then to customize, go into the source tree and update whatever you want to update, and rebuild. You'll see there's already a debian/patches directory where you can drop patches to apply automatically.
I tend to just keep the "debian" directory in source control, so I can take a fresh upstream tarball, check out my debian rules into it, and kick off the build.