5mm plywood here, but I still need to do another one, since the one slot is in the wrong place.
For heat retention, I made a 1/2" foam piece that fits under the heated build plate. the extra insulation helps it heat up faster, and saves energy. I think it was a Teaching Tech article on using foam for a bed insulator that had some good numbers on the heating times and energy savings for using a good bed insulator. The wood just gets a bit warm underneath the heating plate, even at 100°C