Adjust track in cast iron sander top

[fred]

I bought a mitre fence to use in the track in the cast iron top to a belt s ander. The track in the cast iron is fractionally too narrow (less than 1mm ) I'm looking for an easy way to widen the track without losing too much a ccuracy when using the mitre fence. I had a brief try with a narrow block o f wood and some emery cloth but I didn't seem to be getting any where.I als o tried a flat file but it was difficult to hold correctly. The mitre fence has width adjustments screws otherwise I'd try adjusting it 's width