The upper reciever is the load bearing part on an Armalite, but it's also not the regulated part. The lower receiver is, and all you need for that is a box that holds the trigger components in vaguely the right place, a hole that lines up the magazine, a hole to stick the buffer tube into at the back, and some way to nail the upper receiver to it.

You could carve an AR lower out of wood if you were dedicated enough.