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.
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!