Yep, going through that right now with my O Gauge layout boards: Salcombe progress updateThanks, plenty of mistakes though.
Mistake #0001 - Always pick your own timber.
Mistake #0002 - When you buy a square make sure it's square.
Went to Bunnings thinking I would pay $46 per 4x8 sheet of 12mm ply. Every sheet was warped at rates that Captain Kirk would be proud of.
I ended up purchasing 'structural ply' at $71 per sheet. A lot more expensive, but a much better product.
Also agree on the square: cheap knock-offs from Asia are usually not square. Get a German or Swiss product. Make sure it hasn't been dropped!