So I thought about a special fixation for the transport plate,
in which it is clamped under a steel ruler, which allows me to align the other holder at another ruler and could adjust the upper and lower opening with appropriate placeholder strips.
After that I've removed the bottom placeholder and gently dabbed the front part of the left bracket with MEK, which held it in place.
Then I've turned the plate over, carefully fixed it again, and then glued the outside of the bracket with MEK.
And after pulling out the upper placeholder strip, this bracket also sits firmly on the base plate.
Now I can think about how I can glue the
Clevis plates to their smaller
base plate so that the finished
Clevis then also fits into the small opening under the brackets,
what should be at least as complicated and exciting.