Slot machining

I'm trying to machine a couple of spirals on a 2mm brass plate, see image, using 1 pass. The slot width is 2.5mm and i would like to use a 2.5mm cutter to machine the slot.
However, because the slot was not produced using the slot sketch in Solidworks i have to use the pocket process in Solidcam. This requires the cutter to be slightly smaller than the slot and has to make 2 passes to complete the machining. I can restrain the plate on it's boundary and through the holes at the centre of the spirals but after the first pass the spiral is free and chatters into the cutter resulting in poor finish and broken tools.
I would like to use a 2.5mm cutter to produce the 2.5mm slot but can't find a method without deleting the code for the second pass.
Any ideas?
Bob