That's definitely light leakage. In my camera repair box I keep an old black rubber mouse mat. With a cutting knife and a ruler, it's easy to cut thin strips from the mat and trim them to the right depth to make excellent light seals. You don't need adhesive ; the trick is to cut the strips a little bit wider than the light seal grooves. As you feed the strip in, pull it lengthwise slightly so that it gets a little thinner, which lets it settle easily in the groove, then it will relax again and form a perfect tight fit. No glue needed. Make sure to allow for the little microswitch (or whatever it's called) that tells the camera the back is closed.
The Interslice kit is very good, though Jon Goodman has now stopped doing the light seal kit of strips and foam, which you could use to do several cameras of any make, and instead now makes dedicated kits for specific cameras, which are more expensive.
Far and away the most tedious part of light seal renewing is getting the old seal strips out and cleaning the grooves, not putting the new ones in. This is especially true if the old seals have turned to sticky tar. I use cocktail sticks (the things you stick through cherries or little sausages, though generally not both at the same time) to clean the goo out, then very small strips of rag soaked in methylated spirits worked through the grooves with another cocktail stick to get the grooves perfectly clean. I first used this method on my Contax RTS about a decade ago, and it's still going strong and perfectly light tight. The first time I did it the whole job took about an hour, but that came down to about twenty minutes very quickly. Though at first it may seem tedious, it's actually a pleasing and relaxing job to do on a camera.