Thanks Pete!

I've used SS scrubbers from the hardware store too. These look like crumpled up balls of tight steel coils.
I like this idea because the expansion chamber volume is extended to the screens (first point of any real resistance) which are near the outlet of the exhaust tubes.
I have the Slimline pitts muff. for an OS 1.08 on a Magnum 1.08. (I had to make an adapter/mounting plate.) The expansion chamber is decent sized but there is a noticeable back pressure problem there too. I put 3 to 3 1/2" neoprene extensions on the exhaust tubes but they don't do anything for back pressure. The neoprene does however help soften the sound of the 1.08 by dampening some of the vibration and 'ringing' in the muff.

You may need to double-pin each side on your .90.

Good Luck,
Jeff