my way would be if the Dia 6 mm need to be precise (by hand does not work)

mill the part with the Square 5

make a 4,5 thick nest that should be fit to the lower portion

fix this nest into a fix part of the vice so you already have fixed in the three planes -the center line of the dia 6mm vertical

fix the part in the vice -take a null point in the middle of the diagonals( only at first pc)

make the dia 6 with helix mill

you only need to change the piece carefully and push the green button
(sorry but do not know what means you have)