Estes Black Star Voyager Build, Part 9, The Lower Fins

The slots in the longer V pieces should have the slots flattened to better fit the tabs on the X piece.

With the two V pieces taped together down the leading edges and held in a "V"-

You can sand with an emery board. Or better yet, use a diamond file to fit the X piece. 

Here's a good fit of the X piece between the two V piece sides.

No glue yet!

I found it easier to use smaller pieces of tape to hold the long V pieces together down the leading edges.

IMPORTANT! Looking down from the top - 

The leading edge alignment on the left side picture is wrong!

The picture on the right is is what you are looking for.

The leading edges are even. The final fit of all parts depends on the leading edges being even - side by side.