Hello, If i want to ensure there is no insulating layer atop my design I can should I add a box on the 76:20 layer and cover the whole area? We will be requesting the bare dies, so the RDL should not be present

No, don't cover the entire area with glass cut. Put glass cut only over metal 5 where you want to make contact. The pad-related rules are: (1) 30000um^2 maximum area of glass cut (which is about 173um x 173um if made square), and (2) minimum spacing of glass cut = 1.27um.

Okay, So then I think I won't use that layer at all, as we will need to etch it all away. Do you know what the layer is made of and how thick it is roughly?

This stackup: https://skywater-pdk.readthedocs.io/en/main/rules/assumptions.html#process-stack-diagram is correct except that there is no polyimide (PI1) layer on top. That was a process option that SkyWater had on S8 but which they ultimately didn't carry over to sky130.

great thanks