This is from a couple of years ago, and before I had the laser, but it shows the basic idea. 2 rails welded on to keep everything stable, and so they can be marked for the centreline. Note the plastic wedges packing the RSJs level. Then the other rails are clamped on where needed.
Not necessary to do it to this level, of course, and it could be made of wood too, but all I'm trying to say is that as long as you have a means of checking for level, each and every time you use it, and you can wedge chassis parts (or even the board itself as needed) then you don't need a dead flat surface espeically as the chassis doesn't touch most of the board anyway.
The Dwewalt level I use is listed at £190 ish but I got mine from ebay for £150. I use it a lot, bloody good value and you would be surprised how many car building uses there are for it.
For instance, you want to set up a car, fist thing you need is a level floor. Garage floor, flat? you cannot be serious!
Anyway, shine the horizontal beam over the car, use a rule taped to a pole to find the hightest corner, measure the drop on the other three, then pack those corners up with cheap vinyl tiles from Wickes, almost exactly 1mm thick each. Result, car is on a dead flat surface.
Then you can use the vertical beam to check the camber on eacn wheel. I know there are other ways of doing it, but once you've got a laser you won't go back to those.