shielding - EMI/RFI emissions and computer cases

I have designed a computer case (enclosure) for mini-itx motherboards. Its bottom and rear are made out of 2mm thick aluminium and the sides and top out of 4mm plastic (ABS).

I'd like some advice please on EMI/RFI emissions.

I am thinking of making a few for interested people and the end user will be completing the assembly (install their choice of motherboard, hard drive, memory and internal picoPSU), I'll just provide the case.

I have read this question here:

emi standards/best practices for PC cases

and The Photon very kindly answered another user. From what I gather there is no need to FCC computer cases but I'd like to make sure the user can shield the case themselves if they want to.

So is it ok if I provide the case as it is? The aluminium is punched with some air vents on the rear (4mmx12mm holes), some on the base (same hole dimensions) where the motherboard would sit and some where two 2.5" hard drives would sit (again same hole dimensions).

I've heard of nickel based paints that can be sprayed to the plastic parts (on the inside) but is this safe? Does the paint need to be conductive? Is there a risk of short circuiting the components inside, fire or other? If sprayed, would the plastic parts need to be grounded and to what?

What about shielding tape, is that any good? Would I or the user need to cover the plastic parts entirely? What about the bits where aluminium and plastic join? And the vents in the plastic parts?

And finally how do I calculate the exact area required for the air vents so that it doesn't affect RFI/EMI too much?