Bottom Panel and Spacers

Not much progress today... making the bottom panel and spacers took way longer than I thought it would. I made each panel 9-1/2"x15-1/2" (the exact size of the base) but after I cut them out I couldn't figure out how to line everything up perfectly to screw together. The front and back panels have to be EXACTLY opposite each other. This is critical and there is no room for error. I'm going to sleep on it and hopefully a solution will present itself tomorrow.

Here's what I've got going on right now: