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[Chimera]

15mm is a standard size for Plywood. There are a couple of different grades however I haven't had any issues with ply bowing over 4' so you shouldn't have too much of an issue over 6' with centre support. I also use a single side piece with a 5mm deep channel routed in for the floors/ceilings on mine which seemed to provide adequate strength (my 4x2x2 horizontal enclosure was tested with 20kg downward weight on floors and ceilings with no noticeable bowing).

I generally paint the inside of my enclosures with solid colour and therefore use CD grade plywood with the C side (better grade) on the outside. This gives me the ability to patch and hide the open defects on the D side.