I lamented some 12mm birch ply into a 500mm square piece off camera.  Cutting the edges, and drilling holes, using the forstner bit to sink the heads of the bolts and attaching the castors with split washers to prevent the nuts from loosening.  Alternatively you could use nylon nuts.  I than glued a carpet tile on top with contact adhesive to provide a soft surface to prevent damaging anything being moved.