Amplifier Enclosure Resonance

I used 2 Walker resonance control discs on the unperforated strip down the center of my Ref 150. It damped the vibrations exactly as ordered.
Last year, I elevated the amps well off of the floor and removed the top enclosure to get better ventilation. But if the top must stay on--I highly recommend the discs which fit well and work well. 

Did your amp sound better with the cover removed, compared to cover on, Walker devices on?

By three inches "wide", do you mean thick or high? If so, look for a VPI db-5 Magic Brick, whose dimensions are about 2" H x 4" W x 5" L and 8 lbs. It damps physical vibrations and absorbs EMI/RFI. Echo Audio in Portland Oregon has one for sale on their website, no price listed. Should be around $50.

FWIW, I'm a fan of covers off. It sounds better in my experience.

I will listen to the amp cover on vs. cover off. It should be an interesting experiment. Maybe I'm fretting over nothing. But long-term operation with the cover off won't work in my home. Cats and smaller children.

The amp has two rows of ventilation holes with a center strip, which is solid, that is 5" wide. I could likely accommodate something 4" wide. I thought it imprudent to have something cover some of the holes, especially if something were to melt and drip inside.