Well, this evening I decided to try riveting the spacer blocks to the rear half of the carry through spar. This is the fourth time I've tried doing this and haven't gotten a good result yet. This time I decided to try the c-frame method.

Gah! The heads smooshed to the sides. The problem is that after 4 drillings, I've enlarged the hole slightly and now I need slightly longer rivets to fill up the void. Unfortunately, that means they want to tip when being driven. Maybe I'll just put screws in here instead.