Designing a custom ultimaker. Question about Z axis

[bryamatt 0]

Hello, I'm currently working on a custom design of an ultimaker to make use of some parts I have laying around. Going to use 2" angle iron as the frame, and the printer is upscaled to have a print area of 300x300mm. My question is about the Z axis, still not sure what material I will use, but by upsizing to a 300x300 bed should I anticipate any issues with the bed sagging? Would it be a good idea to design for 4x 12mm shafts to ensure stability, or would that just be overkill?

On the note of material, I plan on having a heated bed and eventually an enclosed print area (possibly temperature controlled as well), so everything within in the frame will need to be able to stand the heat.

I am building the printer in Solidworks first and can find a way to upload my design so far if need be.