Something that works for me is stone....(No, not ’stoned’...Stop it. *L*)
You can either replace the glass, or set the amp upon it under the rack if that will work in your situation.
As for the stone itself; get thee done to the nearest (or cheapest) firm that does countertops. They always have ’drops’ from cutting tops to fit sinks, or generally ’leftovers’ of various types, patterns, colors....
We used to have a shop next door that did (and still does, they moved to their ’purpose-built’ shop) and had ’leftovers’ by the pallet load....
...and would throw them away. Too small to be useful....*shrug*
Squared edges, polished...or as fancy as you like. *G*
If that’ll boat your float....;) (Works nice with turntables, too....)