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.

Hi imgoodwithtools:
 I did not notice any difference in sound between top off or top damped with the Walker discs but, in all honesty, I did not conduct a careful before and after analysis. My gut tells me it might be slightly better with the top off, although with the top damped by discs there were no longer any top vibrations that I could feel. My feeling is that if you have no pets or children to worry about and the significant other does not mind---off is the way to go for reasons of both vibration and ventilation. If all 3 conditions are not present, the top with discs is the way to go.

Does anyone make a product currently like this mystical VPI Magic Brick?

I have had great results with Shakti stones. Not too heavy and as well as damping they deal with issues from transformers. The size suits your needs.

Btw have you also tried Herbies tube dampers? Check this thread for what to expect

https://forum.audiogon.com/discussions/tube-dampeners-on-arc-gear-free-tweak/post?postid=1367353#1367353

I'd call ARC.  They do extensive testing and listening to every unit and every product.  If it mattered I'm sure they'd have addressed it.

They even use Litz wiring tho it makes no sense to me...

BTW, I was surprised at the same thing on the top for my LS25 MkII preamp.  it does have a ton of screws holding it tight tho.

 Audio Research has done a lot of testing on the effect the top has on the sound. That is why they went from a steel top to a plexi top on their  Reference preamps. I have both the plexi and a  steel  top for my Ref 5se. The plexi top has a  problem with slightly bubbling from the 6550 tube. I really can't hear a difference in the sound though. Personally, I run my Ref 5se with no top for better ventilation. 

I have some small leather sand bags that are used by artist. I use a pair on the top of my Ref 110 to absorb vibration. Again I really can't hear a difference between the top on and off.

Plexiglass as a top for a tube component? How could that be better than steel or even aluminum or copper for better heat dissipation or EMI/RFI isolation, aside from rigidity?

Lostbears, I have the same experience with my REF5se. I have tried the Harmonic Resolutions dampening plates and found those to prove a more noticeable improvement over the lexan cover. In answer to question above on the amp, you may try these as well. They come in different sizes, I have the small and am planning to sell those and buy a larger size.