Easy enough if you have access to the necessary power tools (usually a drill press or a router). There are a number of companies creating unfinished lower receivers and providing kits for one to finish it themselves. It's also legal in the US.
https://www.5dtactical.com/80-lowers-s/101.htm
https://www.atf.gov/firearms/qa/are-%E2%80%9C80%E2%80%9D-or-...