I thought long and hard about this when starting my baseboards. I used a 6mm ply - softwood block spacer 6mm ply sandwich for the ends of two of my boards, but decided that they were too bendable/squeezable between the spacers. (disclaimer: could be the design, could be the workmanship). I went for 9mm ply for the verticals, but 6mm for the trackbed, as I needed an element of flexibility there. So far seems good, but nothing running yet. I'm wondering about 3mm ply sandwiched for a helix as a way of getting pieces to stick together end to end.