You’ll find many cases where lack of open back slots usually also means there will be components behind the slot that will interfere with a full/half length card.

Also don’t use a Dremel or even a file a hobby knife or flush cutters work just fine. If you have a steady hand then a hot knife attachment for a soldering iron is fine too. The cutting wheel on a dremel tool can run off and the dust it produces can be conductive and short something.