Where have all the good old fashioned machinists gone? :whistling:

Can no-one thread a blind hole in a piece of 3mm plate anymore?

Take a strip of 5/16" brass.

Blind drill the holes.

Grind your tapping drill tip down to square so it can't cut in the middle.

Dip it into all the blind holes, you cannot overshoot.

Thread the holes, (take the tip off you bottoming tap).

Saw to oversize

Screw onto a jig plate from below to cut the sides.

Nip in a milling vice while you cut to height and cut the channel removing your pilot threads.

Quick rub on emery, buff off the sharp corners.

Send them to the customer, knowing full well the threads aren't deep enough.

When he comes back with the improved design, suck air through your teeth and shake your head.
Overcharge him, he is yours forever