The notches on the inside walls are really loose. They don't need to be an exact fit but I think they should be close enough to overlap the insulation and provide a shoulder for the nailer blocks. BTW, I glued the nailer blocks as well.
Keep in mind that this trailer is based on the "Trailer For Two" in the 1947 Mechanix Illustrated magazine. There's a grid in that mag showing the placement of the cross supports. So it was easy to transfer those to the interior panels. But at first, I cut them exactly and when I went to put them in, they wouldn't go. So I had to start wittling them bigger and bigger until I got it right.
Here's a URL that shows the Trailer for Two from that magazine article:
http://www.oldwoodies.com/shoptalk-trailer1.htm
Somewhere I found the magazine article on a free PDF file and when I found it, I immediately DLed it and printed it up. I cannot for the life of me remember where I got that PDF file. If I ever find it, I promise to post the link.
I plan on building another trailer and I'll have learned all the tough lessons on this one. The next one is going to be a breeze but I have to admit, there have been a LOT of problems needing solving here. I bought Kevin Hauser's CD-ROM from
www.kuffelcreek.com. I think that Kevin's book is money well spent. However, this far into my build, I have to say that even Kevin's book does NOT cover all the bases. But that's what this forum is for.
So really, I've gleaned a virtually everything from a combination of the original article, Kuffelcreek and this forum. If I had left anyone of them out, I'd have been in deeper trouble.

I am NOT a complete idiot! Some of the pieces are missing.