be carefull with the 4,5mm plate.
ask to your supplier which are the standard size for brass.
probably the 5mm exist. they have to mill all the plate to 4,5mm, then complete the cnc work.
(that means that a 5mm plate will be less expensive to a 4,5mm plate)
will the plate be fully milled or water/laser cutted and then milled? check the minimun radius your supplier can reach by milling.
the top part (as showed in pictures) may be laser/water cutted, adding later hole and thread.
if you want a chamfer on top this will drastically change the price.
when you design a components keep in mind how it will be made