To be clear, are you talking aluminum or stainless steel? Billet just means a block , so what material are you working with?
Would think SS would be quite cost prohibitive to machine from billet stock for something like this, not to mention how much longer it'd take to machine due to its inherit lower speeds required...then there is the price, something like 304 SS in that size would be $$$!
If using that material, would be better off with a welded/mechanically fastened assembly of machined parts and fittings.
Sure would be nice to have something like that though, probably would last longer than the car, lol.