Microphone capsule to GPO 21/A adaptor

I was turning an old GPO 874 rotary phone from the 1970's into an audio guestbook when I realised that the original GPO 21/A microphone wasn't going to be good enough quality. So I salvaged an electret microphone capsule out of an old headphone/microphone set. I didn't want this capsule to be free floating in the handset, so copied the original mic geometry with a space to house the capsule.

This should fit an 8mm diameter by 5mm high capsule. If there is demand I might alter the design for other sized capsules.