Outer Limits 4x4 Board

After a modest spring lift of 2 inches in my SWB (using kings springs) the rear is a little lower that i had hoped, although the sills are level. The right rear hits the flare - 32 bfg muds.

Is there any preference to brands of spacers with respect to price and compound? I only want about 15-20 mm. I'm sure this won't affect the travel of my 2 inch koni shocks or add driveline vibration.

Thanks,
Dunc.