Basmundo, you are way, way, way, way, way, way, way overthinking this. I know I have said you are overthinking this before. Cutting rubbers is really not a big deal.
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I want you to glue a piece of paper onto the cap of something like a large mason jar and then cut it. Do that as a test run. Cutting rubber is only a little more complicated than that. If you wanted another test, you could glue a thin piece of cardboard onto the lid of that jar and then cut. The rubber is a little harder to cut than the paper and a little easier to cut than the cardboard.
If you are really, honestly worried about messing it up, find someone who has done this before and get them to do it for you and watch them do it. Then next time you will have seen the process in person. What ever version of cutting the rubber they choose will be a fine method for cutting the rubber.
But don't obsess about this. It is only rubber. And again, if you are worried, get a cheap blade like the Yinhe Galaxy 896 ($16.00 USD) and cheap rubber like Dawei 2008 XP or Super Power ($8.00) and do a test run. Heck get 4 sheets of cheapo rubber and do 2 test runs before you glue the Tenergy.