Its not very clear what you are doing exactly, obviously is one thing machining 200mm thick part or 10mm thick part. But from what i have seen and know- what you are doing is done usually either using a vice and parallel plates below part, either 2 piece vice for long parts, parallels if needed and a combination of screws and clamps.

Skimming flat on mill is achieved by fly cutter, for your case best would be custom heavy duty fly cutter so you cut at once as much wide as possible. meaning made so that is reinforced with disc.