What He Said ...
Bill's comments + 1.
Depending on where the spars came from, and how they were assembled, there can be differences in their thickness (in .000's of an inch).
In order to insure your spar will fit the bulkhead later, you need to determine their exact thickness, each one, as they may not be the same, then add (optional) a few thousandths, and make a "simulated" spar out of anything that is rigid & stable enough to maintain its dimension during its use - factor in where you live in relation to the change in moisture content during that time. I live in a very dry environment - so not much of an issue with me. I used kiln dried oak, sealed with some sort of "poly" finish.
One thing I learned from previous experience, is that the "spacer" can be difficult to remove when its job is done. What I did to offset that, was to build into the overall thickness required a .025 - 2024-T3 shim that had a "tab" on the outside long enough to act as a "handle" for ease of removal.
I also made two "half" sets of simulated spars (each 1/2 long - just like the real thing) instead of one spanning from one side to the other, to make the eventual removal easier.
When it came time to remove them, I took out the aluminum shim first, then removed the spacers, which because of the .025" clearance, were very easily pulled out.
I've seen some ugly results when the spacers were not made directly from measurements of the spars being used - not saying it will happen, just that can.
HFS