Here's my vacuum "chamber". It's the little black box with 4 hoses going into it. Not very noticeable! Since I have four two-barrel throttle bodies (made by IPSCO), I only have four vacuum hoses. The thing with the connector on the left side of the black box is the MAP sensor. The silver thing to the right is the drive-by-wire unit.
What would make me nervous about a cast-in plenum is, who decided what volume it should have? There's a bit of science there. The volume can have an effect on how the MAP sensor reads. If you're just running a MAP sensor, the plenum doesn't have to be very large. If you're tapping into the plenum for an IAC and/or for a power brake booster, you need a larger plenum.