I made this to flush cut structured sides in the mold. I haven't tried it yet but I think it will work. It's made for the Harbor Freight Drillmaster trim router which really is a serious POS but it's $27. You can use any 1/4" bit with it, I have a 3 blade CNC leveling bit that takes inserts to use.
Not sure who would want to use this but basically the idea is that you can cut something flush or pretty close to any surface the base is resting on. There's also a little groove in the bottom so you could potentially ride a rail and cut it down as you go.
Anyway, here it is. Maybe useless to all.
Print it in PLA, it is stiffer than PETG I think. Tree Supports work well.
There are little slots for the cheap Harbor Freight adjustment handles.