To follow up on my hunt for a powder compact refill, I have successfully fitted a Tunnel pressed powder cake into one of the bigger round compacts.
To fit it in, you need to remove the plastic casing, while leaving the metal inner tray and powder as undamaged as possible.
I used a variety of tiny metal tools to Jimmy it out round the edges. Also, if you Peel the label off the back, there is a tiny hole you can push something strong through to lift the tin out. The tin is glued in the centre to the plastic, so it takes a patience to get it loose.
I measured the compact tray before shopping around for a refill, so I was able to just slip it into the compact tray. As this compact doesn’t have a mesh sieve to sit over the powder and protect the mirror, the powder puff (from H&M) fits perfectly over the top.