After much cogitation, and asking for advice on a message board, I decided that the best way to go was by the instructions, and build a box beam. My first attempt was with 3/4" CDX plywood. I built it crooked. I thought that particle board would be more dimensionally stable. It came out nice and straight, and weighed a ton. While I was trimming the ends, it fell to the floor and broke in half. I then got a strong urge to try it with an external strongback, but I'm resisting.
I resorted to using my most-hated building material: OSB board. It seemed to be the most dimensionally stable.
I'm using the I-beam from the o.I. build as a table, and I made plywood brackets to hold it up.It looks straight.