Space inside the bended parts is necessary to insert and keep inside, some smal plumbs. They help you producing kind of a sound when the cap fishing lure is moving in the water.
Also flat ends are necessary, cause it's in those two flat parts that I'll make the couple of hole where to fix the treble hook and the main line.
Do not put too many plumbs inside, they will frooze the motion of the cap fishing lure ... three is a good number in my case, but it depends on the river, lake, etc... try the cap fishing lure and in case add or remove plumbs.
Topwater vs shallow running
In case your cap fishing lure is working topwater and you need it diving more deeper, as a first step you can try changing the plumbs inside.
If testing it you see the motion is not good, instead of adding bigger plumbs inside, add a single plumb (a couple of grams could be enough) in the main line, 12 - 16 inches before the lure (30 - 40 centimeters).
Note: probably this ring is too big! I just run out of rings!