Particle board is glue and sawdust. It is not strong at all, tending to sag. Just look at what happens to cheap kitchen cabinets and bookshelves. If it you build a four sided box wall tube out of it, it will still sag between the supports it rests on. Especially with the weight of a boat upon it.

In fact, a 2x4 isn't all that spiffy either when left unsuported over a span of say 10 feet. Try 2x6 instead, and if you really want it strong, make it like an I-beam using several pieces of wood. Then it doesn't have to be heavy pieces of wood.

While it's easier to set up the forms on a perfectly straight strong back, it isn't essential. Using lines and other measuring techniques, you can set the forms onto a squigly strongback in a nearly perfect manner. The key is that the strongback must (must, must, must) hold its shape. If it doesn't, you're wasting your time.