i just built something that was intended to be safe indoors. it is still a bit too heavy and big, but i saw no way to make it smaller.
it is a 3" Owl frame built arducopter that uses kakute f7 stack. to fly altitude hold correctly indoors you need to setup a lidar with a preference over baro, i also use benewake tfmini.
tfmini has to be setup correctly as well, there is a thread about this.
to make it use lidar you need to set some options in the EKF to set lidar as a preference. baro will never work indoors correctly. it has to be shielded and covered by foam.