Scaling will not work.
Yes the supports are available on thingiverse. I did not make them but really it doesn’t take much anything you can get in between the roller bearings would be fine. Every midspan support increases rigidity 4x’s (if I remember that class right).
Unless you have a specific need for a machine that big I would start much smaller. Building it huge to to build it huge isn’t a good idea. Something that large has it’s own set of issues that you might not want to tackle from the beginning. It will make everything much harder. Getting a first layer to print on something that large would be pretty difficult not to mention some insanely long print times. Lots of reason large machines are not commercially available for the hobby market.
If you could get one of the dimensions smaller than it gets much easier. a giant square isn’t the most rigid configuration, but a narrow rectangle would work much much better.