Here you see the nuts, washer and spacer that I used to make a temporary spacer. My original plan was to use particle board, as suggested by Vans. I couldn't find the right thickness, so I tried layers of medium density fibreboard and plywood, plus a thin piece of aluminum. This gave the thickness I wanted, but I found that when I torqued the bolts to hold everything snugly together that it crushed the spacer a bit, and the two parts of the bulkhead were too close together. My wing spar wouldn't have fit.

I ended up sliding 5/16" nuts, plus one washer, plus a 0.040 aluminum spacer over the 1/4 bolts. That gave me a spacing about 0.003 more than the wing spar thickness. Note: commercial nuts are not all the same thickness, so please measure things if you want to do your spacer this way.