You're starting at 600m???

That's nearly 2000 feet, and you're looking at a pressure of nearly 900PSI.

I don't know a source for your materials, but I do know that you should start much, much shallower than that, increase your depths incrementally until you find your sub's maximum depth - if you drop it to 600m, and then pull up a wrecked mess, you don't know it it failed at 599m or 5m.

I'm just guessing here, don't listen to me unless some knowledgeable people can verify it. Polycarbonate type materials are used in bullet-proof glass. I'd also say If you're not going to put a living thing in it, fill it with an inert oil, as oils are unable to "compress" If you want to be ambitious, here are two words for you, pressure equalization. I hope some of this helps you! Also, when you decide whatever you're working on is secret no longer, might you share it with the community?

700 to 900psi is about the pressure you see inside a CO2 tank, so figure as a rough guess you'll be looking at something about as "sturdy" as a CO2 fire extinguisher (de-certified extinguisher might even be a good starting point.) Any window is going to be "quite thick." You can think about some sort of scheme to internally pressurize (with gas) to match the external pressure. Be careful, though; things with a couple hundred PSI inside make for nice explosions!