Thinking laterally:
High vacuum means not a lot of stuff in there; except what you are sputtering onto your lenses.
Not a lot of stuff means not much to cause oxidation.
Why not use mild steel?
One problem with stainless I think you are bound to have is distortion. Any sheet/plate is full of internal stresses and 600C is high enough to start them moving.
Whatever material you use I suggest you consider getting them laser cut roughly to shape and size in plate that is around 8 to 10 mm, and have them annealed before machining. It may even be necessary to rough machine, anneal again and then finish.
An open mind is a virtue...so long as all the common sense has not leaked out.