When I run into something even close to this these days I just look for a base to put on the gun that has the correct size picatinny on it, and mount to that. Some are made just to raise mounting a half inch and they usually tighten down more than just the rings do by themselves. A little wonky looking but it works fine.
Go to Amazon and type in "Picatinny Rail Riser".
Go to Amazon and type in "Picatinny Rail Riser".