You might play with the idea of necking down 308 brass. When you neck down, particularly from .308 to .264, the necks naturally thicken, as the brass has to go somewhere. If you play around with a few pieces of 308 brass, you may get a combination of neck diameter and bushing size that works for your chamber. You may have to try a couple different brands of 308 brass to end up with the right thickness when done.