Be careful modifying your tools! Depending on what the brand of tool and the material the tool is made from, this could result in a tool that has limited life. If the tool is chrome plated (shiny silver looking), filing too far could break through the plating and cause the remaining plating to flake off. The metal under the plating starts to rust, resulting in tools that are not really usable.
I agree with Bob, go back to basics and use your modeling spoon. An alternative idea is Tandy Craftool Pro Modeling Tool Small/Large Undercut Item #8039-05, which has a very small tip, great for beveling tight places. CAREFUL: this tool is also used for undercutting, so a little pressure goes a long way.
Hope this helps
Mike