I wouldn't use a mast section, that would be way too big. A regular H14 or H16 boom would be easy to find and should work. If you are looking for a stock extrusion, the H17 boom is nice and small. They are pretty hard to find used, though.

If you look at the A-cats, they have a really small aluminum, fiberglass or carbon boom. The blocks attach where the leech attaches to the boom, so the boom is really only there to hold the sail out where it should be and to give you some foot adjustment. They also have a semi-circular or elliptical traveler, so the force is always pretty much straight up and down compared to the angle you get on boats like the 14 and 16.

The Hobie 14 Mylar square top sail I ordered from Sabre came loose footed. I expected it to be footed like Bob's, but it wasn't and I'm pretty happy with it. I can get a really nice shape out if it after a little boom modification. I took the outhaul adjustment from a Prindle 16 and installed it on the 14 boom. Works great and is easy to adjust. Just another idea.