Vic Balshaw
Major General
Dave, I'd be inclined to razer saw the base off at the lip (Pic 1), then drill small holes from the inside (Pic 2) so that you can cut the middle out with a sharp knife or a fine key hole saw (Pic 3) if you have one. Then you can either saw back the surplus resin rim (Pic 4) to a point close to the kit part and then file to shape or just file back the rim to the shape required.
Hope this helps or gives you some ideas.
Hope this helps or gives you some ideas.