Well, small update here. I ordered a piece of tempered glass from a local company a number of months ago, but I didn't measure close enough and the holes in the resulting piece were not quite right. It was expensive enough I shelved the piece instead of trying again, but then a friend of mine got into 3D printing. It took a few tries, but he printed some small adapters that also function as standoffs for additional GPU breathing room. I ended up using rubber washers from the unused HDD cages with this case to secure the glass safely. I'm very happy with the end result!