I decided to cut the large block in halves to make a tall chamber with a small base. It will be milled in the following days when the workshop guys have time for it.

The lid will probably look like this:

The reasons for two thin glass plates are 1) thicker glass is harder to cut and b) old picture frames from ikea! The silicone will have to withstand high temperatures but there's that stuff for fireplaces that will handle 250°C with ease, which is more than I expect anywhere around the lid.