That is some very good advice about getting arctic birch ply ripped down to "1 x4" size. I think that is the best option to get stable lumber in the modern USA. Dimensional lumber offered these days is terrible.
For decades I have been salvaging wood removed from buildings being remodeled/torn down. Most of these buildings were built in the mid-1800s and had actual 2" x 4" 2x4 studs cut from Old Growth yellow or white pine. Grain counts going over 150 years, just for construction studs! Most of this wood I use to construct furniture, but am considering some for the upcoming layout construction.
I was also lucky to acquire the wood used for shelving from a local hardware store that closed. These boards were installed as shelves in the 60s and measure 3/4" x 15" and 3/4" x 17". One solid piece, not multiple boards glued together. A few at 14' long. Some have grain count over 80 tears. And since this wood has been mounted horizontal for decades it is fully dried and stable. You just cannot get wood like this anymore. Most of these boards I have turned into table tops, but I have a few ear marked for layout use.
My point is that modern dimensional lumber is more or less useless for any permanent quality structure. It is usually very young, very wet and bound to warp. Even if you go through 100+ boards to find straight ones, after six months to a year, when they start to dry they will warp.
I am about to start construction of my layout benchwork. I plan to construct my layout in transportable modules with furniture grade wood, with an eye on making the layout last for many years, so this discussion on plywood got my attention.
With all that said, Tom's suggestion of ripping arctic ply to strips is the ticket to stable bechwork.
As for the top, I'm planning on 1/2" Baltic Birch. With a structure spacing of around 16" it should be solid enough if I have to climb on top of it.
And a few questions for Tom about his benchwork. Why did you install the gussets in each corner? Are they really needed to reduce flex? And what are the gussets made from?