Depends on how much cutting you want to do
Unfortunately, that is not such a simple question as other important factors are amount of cone travel and clearance of the actual speaker itself behind the interior panel will have an impact, and do you want to trim some of the interior panel to make it fit. So this could almost be rephrased as "what is the maximum spacer thickness" and yet the clearance issue remains unresolved in this aspect also.
One solution uncovers another problem.
EDIT: This could very well deserve to be the stickiest damn thread on the site!
Mods, seriously, this should probably replace a thread or two at the top since it DOES happen to reference some of the current stickies that maybe don't necessarily need to be there anymore. Just my input.