Had a go at making another wedge. Previously made a 4mm at thick end but it did crack from lack of matting & filler. But it's still useable.
This time I made one at 6mm with filler & 2 layers of matting plus an extra 2mm on thickness. Defintely better.
After laying down the last layer of matting I just poured over the rest of the epoxy to keep the surface level & smooth.
Used the same mold with vaseline wiped around the inside. Works great.