how do i make hole in stl fusion 360 hobby?

[newns]

super new to 3d modeling. all im trying to do is take a vat cover stl cut out a rectangular hole near the screw holes so i can just slide it under the bolt heads rather than have to take the bolts all the way out every single time i want to remove or install the cover.

when i convert it, and use the extrude tool to make the cut, the faces that lining the cut area are a sickly yellow which i assume means there isnt anything there??? 

i attached the stl file, an image of what happens when i try to accomplish my goal as described, an an image of a top down view of what i want on the right and what it starts as on the left. id also like to fill the four holes in the middle but i have no idea how to use this software and after spending hours looking at youtube videos im even more confused as many of them are outdated or require paid features to do what im trying to do. 

[Warmingup1953]

I have quickly repaired, created facegroups and converted to a solid body. Have you started on any training? There's inbuilt as well as fantastic online resources.

EDIT: It would be a good semi-beginner project to start within Fusion and design the better Mouse Trap so to speak because this one has very little fall for drips: