Can the electrical board be placed directly onto the aluminum plate or do we need to place a plexi glass plate in between them?
There are no specific requirements for what you mount your electronics to. However, please reference R27. Note that most, but not all, of the electronics do not ground to the case, and thus could be mounted to anything (if set up properly). Check to make sure on yours, though.
Oh, and one other thing: Use Lexan (polycarbonate) instead of Plexiglas (acrylic) if you do go that route. It’s going to be a pain to replace all that cracked plexi… Or just use wood for the bellypan from the start.
This year you should be fine with straight onto aluminium. The thing that you need to watch out for is “grounding” (and energising) the frame. This happens when there is an electrical connection between any cables/terminals, and your robot frame.
A quick educational test you can do is to get a multimeter and check for continuity between the cables/terminals on your components, and their casings (the surface that will press against the aluminium). If the multimeter shows anything smaller than 10kohm than you will need to place something non-conductive between the frame and the component (Hint: everything should be already ok). This is also the same test they will do at competition.
Least year our electrical platter was a combination of two thin aluminum plates and mounting electronics directly onto the frame of our robot. We had no issues with this design. The one thing worth mentioning is that we created a field of electrical tape on the plate by cutting a large square out of strips. It was used to insulate the Crio from the aluminum plate. We also mounted the Crio with nylon screws to absolutely be sure that the whole area was insulated.