Try these Hudson silicone feet. They work similar to sorbothane but don’t deteriorate and don’t leave any residue. I use the larger ones under my subwoofer and they work really well. They come in a 4 pack so you’d need to order 2. For better stability, I’d advise putting them directly under the case, next to the feet, not under the feet.
Alternatively, if you want a little more height, try these metal feet. There are several models on the Amazon page. I’ve used model IF4023B. Like the ones above, these come in a pack of 4.
