Upside down and with weights put in place, or screws and glue, and then weights.
My favoured construction method now uses 9mm ply ends (100mm deep), 6mm tops and sides (100mm deep), with a 9mm central "spine" 70mm deep running the length of the board, down its middle.
I start with one end screwed and glued to the top. The end fits within the sides, by the way, so there is 6mm at either side. I then add the central spine (this can be placed a buit off centre if you know where the track will be going), which is 18mm shorted than the sides, with a screw through the end piece into the end of the spine, and a few small screws (and glue) along the centre line of the board. Ths obviously involves quite a bit of turning things over, but it pulls the centre flat.
The other end is then added, again screwed and glued. After this, the board is put upside down onto a flat work surface, and the side pieces glued/screwed into place into the ends. You can add a few staples to these joins if you wish, flipping the board over briefly. At this point, I put the assembly face down back onto the work surface with a few piles of books/magazines or whatever on the underside of what will be the baseboard surface, to hold that flat, and place either another board if there is one, or the top and materials awaiting assembly if not, on top of the frames to hold things down. I then let the glue (Resin W - won't use anything else for this work, on the advice of a joiner) set hard by leaving it alone for a few hours - have usually had enough of woodwork by this time, anyway!
Next day, I can then insert 50mm deep strips of 6mm ply into the structure to provide triangulation and cross bracing, again putting weight - a bound set of magazines is enough for this - on top of these pieces, but an hour should be enough for this stage of the process. You can also add bits of 18mm square timber into the joints, if you wish: I think this is useful along the sides most of all, and you can add these to the side pieces before you add the side pieces to the tops. (This is an idea I have just had, and wish I had had before!)
I have made boards up to 4' x 3' using this method (the 3' wide board had two central spines, but those up to 2'6" only had one) and it is surprising how few diagonal pieces are required underneath - three or four each side of the spine are enough. The resultant boards are strong, fairly light and resitant to twisting. Barry Norman tested one by standing it on end, with his feet on the end piece, and trying to twist it. There was a small amount of movement, but a lot less than with his beam and spacer ideas. He did mutter something along the lines of, "Maybe I should re-visit ply boards".
Trevor Nunn simply used 4mm ply throughout, using weights whilst glue joints set, with the boards built unpside down on a large table: see MRJ 86 for a good shot of the general idea. These boards have remained flat and stood up to the rigours of over 15 years on the exhibition circuit, and being stored in a garage between shows.
I once made the mistake of not thinking this through, and whilst I put the framing on, etc, I forgot to turn the tops over, so ended up with a concave dip which was much hard to correct than a convex one - the boards went to the tip in the end, but not until I had cut and laid about 1,500 sleepers. (Only noticed when I came to add the rails.)
Sorry, a typically long post from me (someone once claimed I held the record for length of post on RMWeb!) but I hope it helps.
I am currently without layout, so cannot provide photos.