Another tip.. once you slit the tubing and place it on the lip of your kettle / fermenter, overlap the ends, then take a new razor blade and cut through both layers at the same time. This ensures they'll be as seamless as possible even if your cut is not exactly squared up. I also made sure the tubing was not being stretched before making the cut. The result is a really tight seam. I still bridge the seam with a narrow strip of saran wrap for added protection.
That all said, once fermentation kicks off the beer should be protected by a layer of CO2, so the seal really just needs to keep stuff out vs. being airtight. You'll know you have a good seal if there is activity in the airlock.
Let me know how it goes. I have 2 running right now. One with BierMuncher's Centennial Blonde and the other with a variant of EdWort's Pale Ale. Here's a picture of my "Laurel and Hardy" fermenters with a total investment of $65 for both.
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Curious about your brew sheet. Happen to have a PDF or DOC you could share?