If you really want a titanium case, you should engineer it to properly make use of the materials strengths.
Aluminum works well in machining operations because it's easy to mill, and it's softness requires more material/support. Titanium is more of a hardened steel and works better as thin tubing or welded plates and not so much as a machined chunk. Can you machine it from a chunk, yes, of course, however price will be insane, especially when you consider more than 90% of it will end up as scrap. You do not need the walls or bottom to be as thick as aluminum, it's not an efficient use of the material, it's properties, or your money. A better option for a titanium keyboard is either make it from welded plates of titanium, or cast it, either method results in far less waste. You could even weld plates or make a casting, then machine it, again generating far less scrap (casting then machining is what Vortex does for their aluminum cases). Another interesting option is weld small diameter tubes of it together, building a cage for a case (look at race car tube chassis for inspiration).
Unfortunately, casting it is probably beyond most peoples ability, and unless you know someone who regularly welds Titanium, that's not so much a good option as welding Titanium is difficult, it becomes contaminated easily making for a very weak weld. Then again, for what Titanium costs, finding a welder capable of it is probably cheaper than any other method. My advice would be to just have a tub made from sheet titanium, then drill it and make standoffs and such from plastic or aluminum rather than trying to make it entirely from Titanium. I would also advocate having some done in steel first, because if you get it wrong, it's gonna hurt.
