To get a fully enclosed chain case for a front and rear derailer system you'd need to build an awfully big box, or one with a couple of protrusions. And you might encounter some interference issues between chain case and spokes up by the cassette.

Still, it's not like it's rocket science. Getting the chain reasonably well protected from above is easily doable. Start off with the biggest chain case you can find and either some thin sheet metal aluminium, or something like a sheet of Lexan from a R/C hobby store. A roll of assembly grade double sided sticky tape, some hand tools and you're all set to start crafting. You might want to use cardboard to make up templates first.