My Antec SX11040 both side plates have an area that I was able to cut the foam and tuck the foam under metal to hold the foam in place. I also used two sided tape in the middle to hold it flat. The top of my case has plenty of room.
CLOSED CELL SPONGE RUBBER: Very dense and durable. Used for gasketing, sound barriers, weatherstripping.
OPEN CELL SPONGE RUBBER: Highly compressable and resilient. Used for cushing, packing, insulating.
Now for the bad news, OPEN CELL SPONGE RUBBER is not cheap. Most refrigeration shops carry a lower grade of foam that will work for your application and for a lot less money.
This is the softest of all rubbers,
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