The wings and upper and lower rudders glue on in a cruciform, 90 degree spacing. The wings are set back farther than the real X-15 for stability.
The strakes glue over the top of the wings. Notice the gap at the root edge of the strake.
For the best fit, sand an angle into the root edge.
The inset picture shows the improved fit at the root edge.
Here's the lower fins before and after the primer/filler was sanded down. There's very little primer left in just the remaining grain pores.
The rear fin root edges are also sanded at an angle to set under the fin and strake.