I file a flat on the thimble the same width as the bottom of the rib where it will be attached. Then I use my vernier caliper to measure the thickness of the thimble where I filed it.
Then, I file a notch in the rib to accept the thimble, using the extension bar thingy on the small end of the vernier caliper as a depth gauge.
This way, the depth of the notch in the rib is the same as the thickness of the thimble at the flat, and the hole in the thimble will line up with the groove in the rib.
Then solder. I usually tin first, but it probably isn't necessary.
Dale H