The OP's levers don't have a split clamp like most flat bar levers do. They use a wrap around band and pull up clip/screw. So there's no slot to pry more open. But there is a tool that if the OP had access to would help. Park made a band expander tool. I didn't mention it because it's not common these days, almost no home mechanic has one and many shops don't also. Think of a pair of pliers with half tubes welded to the ends. When the handles are squeezed the ends spread. Insert the half round cylinders into the lever band, squeeze the handles and the cylinders expand out and increase the band's roundness. I've had a pair for decades, never found them to be needed often back in the day and haven't used them for years. I searched for a on line image or listing without luck as they are long since discontinued. Andy.