The first prototype was just aluminium foil, tape and hope, but we wanted something more solid so we built one out of n°100 copper mesh and some 2020 aluminium extrusions!
The door of a microwave typically doesn't form an RF-tight seal. Instead there's a groove that forms a resonant trap at the microwave's operating frequency. So it'll probably block 2.4-GHz ISM-band stuff like Bluetooth (I don't actually know how wide the trap band is compared to a BT or wifi channel), but outside that band all bets are off.