As an ex-audiophile
I am curious as to why y'all are wanting to run spacers? Are you concerned that the magnet would be resting on the panel, or?
I used a foam tape in-between the speakers and the panel, put sound deadening material on the panel and installed everything. Sounds as good as can be expected with so little space available and as much power I am pushing. BTW, I am using Alpine 6.5's.