Your two caps should hopefully sit on top of each other nicely. If they don’t, then sorry, you went a little wrong. When gluing these caps together you should use a strong 2-part epoxy. I only had superglue at the time which isn’t as strong, but it’s held out quite well so far.
MAKE SURE THAT THE CAPS ARE THE RIGHT WAY ‘ROUND! You can test this by keeping them in position and screwing the camera and lens on as shown in the picture.
When you’re ready you can get gluing. The picture looks a little risky with superglue near the camera, but I was quite careful and used the weight of the lens to my advantage. As soon as the glue started to set I instantly took the lens and converter off and left it to dry for an hour or so.