Board is 4.5mm thick, cut is to 4.6mm.
Use 1/8" double-upflute bit
Board size: 400.5mm x 400.5mm

Let's try: Two sheets of plywood. 
 For text/symbol: 1/8" bit.
 For holes, use 1/4" bit
 For outside cut, use 1/4" bit.

   (Box should be SLLLIGHTLY SMALLER)
 If compensating for a 1/4" bit, total OD = 412.6 x 412.6 (399.9mm x 399.9mm ideal.)