When this happened to me I took a different approach. I contacted the manufacture and told them what had happen. I asked for some scrap under seal material. They sent it.
Then I laid some thin wood panel material 2 inches wide up inside the current under seal and then ran in some self tapping screws through the current under seal. Then installed the new under seal by also using self tapping screws through the material into the wood.
I finished up with using enternabond tape on the crack. Never had a leak. Neither did the new under seal ever fall off.
Bob