The face frame on a filco is 5/16" above the plate at the front and 3/8" above at the back. The curve is subtle enough that it's only important as a design queue.
That's lots of space to work with. In regard to how it's been done before, screws down through the plate ears to hold it to the base, screws up in another location holding the top frame and other layers to the base.
What material are you looking to work with for the case?
I thought about the case more tonight. I wasn't sure what you were talking about when I first read it but after thinking about it the case more, I was going to come back here and ask lol. Since the difference is so small, I'm not going to worry about it, like you suggested.
The only problem with your suggestion is that base is a 1/4" flat plate, nothing more. So I'm not sure how to secure the plate to the "middle" without milling the top.
I'd like for everything to be aluminum. 1/4" for the base, 1/2" for the "middle" and 1/16" for the plate.
My plan is to have the middle, base and plate laser cut a bit bigger than final size because the edge quality is poopy (except the plate since the deflection on 1/16" isn't nearly as much as it is on 1/4" and 1/2"). Then have them sent to a machine shop (or treble318, I've already sent him a pm) to clean up the edge, bring to final dimensions, drill the base and then drill and tap the middle.
Deflection:
I'll try to get better pictures up tomorrow. How many ears on the plate should I need? Right now I have 3 holes on the short side and 5 on the long side. Would you think 1 plate ear on each short side and 2 on the long side be enough? That would leave 2 screws per short side and 3 per long side to attach the base to the middle.