Did you break the plastic tab?
I did and tried glueing it but it didn't work. Then I carefully removed the plastic spine from the stock cover and fitted it on my tri-fold. That works great.
Just remember that the plastic spine has two catches and some glue on the middle. Carefully lift the middle until it comes away from the cover. Then push towards the centre with one of the ends (bending the spine like a bow) to release one of the catches. Once that one is free, you can easily remove the other one.
Btw. I recently bought the Oberon cover and now can't see myself using the tri-fold any more
