When I had a 23C a few years ago, light was leaking out of the top bellows that slide along the condensor sleeve as it gets moved up and down between covering 6x6 and 35mm.
I made a kluge fix light trap up out of some velcro stiched to the end of a band of black velour. The band wrapped around the condensor cone, and the velcro did not come all the way to the ege of the band, so there was a kind of skirt that rode over the edge of the belows and blocked the light.
The velour came from a black kids church type dres bought at the thirft shop just for the right kind of fabric to cut to make my own 'skirt'. I guess someone when little had to go to grandma or grandpa's funeral when they were little, and this was the dress they wore.