Enlarging the hole of U-adapter

Before the process, I only have access of a simple hand drill borrowed from someone.

The difference is not too small, 3-4mm I measure:-

It was difficult and required a lot of patience.

 

This is close to done, but I would leave it alone first after like 2 hours of hard work:

When this is done, I've to enlarge the mount holes to fix it on the Mark-X mounting holes.  And then I will need to drill two holes on the mounting plate to fix the DEC slow motion control worm block.