its gonna warp no matter what unless you brace it i build subwoofer boxes to. and i use 3/4" mdf or 3/4 birch and i still have to brace them cause of flex so there really no way around it
3/4" mdf/birch/oak/melamine should be strong enough but then again if it gets wet thennnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnnn
it warps