Surprisingly, the cheap memory foam ear plugs can (depending on brand) offer up 33 dB reduction, which is on the high end for sound reduction. That all depends on proper insertion though. Some people have them looking like they will fall out. That isn't doing them any favors. They really have to be inserted for max dB reduction.
That being said, Squib308 just pointed out to me yesterday that humans also conduct sound through their bones (skull), and that's why using just ear plugs isn't great for indoor shooting (which I can confirm). Unfortunately, that means that cans are still the way to go indoors if you want to avoid that type of bone resonance.
Have you thought about padding the shaft to create a higher cheek rest? I think I saw someone put gray pipe insulating foam around their Sub2k for that purpose.