I don't have any holding or cutting suggestions (yet), but I do have a comment on the design; how good is your relationship with the research people you work with?
It seems the design calls for a groove to be milled along the edge of a piece flat bar with a big notch in one piece. This is as opposed to fabricating it from three pieces of flat bar bolted together.
If you take stainless flat bar and machine it along one edge like this it is almost certain to warp; it may warp a considerable amount. This is because when the bar is rolled tensile and compressive stresses are introduced into different regions of the bar and when it is machined these stresses are relieved unevenly.
If you have a good relationship with the researchers you may be able to get them to do some experimenting by just machining along the edge of a sample of the bar to see how much it moves; or you may be able to get them to understand that if the part comes out all bent after you have machined it, it is not your fault.