I second the tape method. I never use the light slide, instead I remove the film holder while the back is still on the camera. My camera also had a velvet insert instead of foam when I got it, but I replaced it with foam. I am sure that it was serviced at least once in its 20 plus years of use before I got it. I would worry more about spacing than light leaks on a new back. Shoot a roll and see how it comes out. I am sure a local camera store can process the film for you in a day or two if you do not have a darkroom to do it yourself.