That was a design choice. I didn't want the hood to catch the breeze when flying forward, which I thought might make the camera shake. Also I didn't want to make filter threads out of nylon - I wasn't convinced that they could be 3D printed accurately. For those reasons, I made the diameter a bit oversized. That leaves a small gap around the inside (behind the lens) to allow for airflow. The frame bit at the bottom has two pegs that are the attachment points for a rubber band that wraps around behind the filter adapter frame. The top has one wide tab that clips behind the filter holder frame, and one that catches the lip of the filter. The prototype in the photo fits very snugly with no play once the rubber band is in place, and I tried it on several different filter widths. I also considered snap tabs that would clip to the back of the filter holder frame at the bottom, but they would be thin and therefore a potential point of failure. Once I have the sample from Shapeways, I'll post photos.
Here's the final design: