Your local waterjet shop should be able to sort that, probably for cash if you supply your own material. Base would need to be removable as said before, internal radius 0.5mm min and width of cut 0.8mm so you get back a nice lump to make something else out of. Our customers often get us to rough pockets in 50mm stainless just so they can use the 'pocket' for another job. Better than turning it into swarf!
All the best.
Ken