As noted in the project description, I want a see through lid. So I took the glass (2 mm thick) from an old picture frame and cut two pieces about 60 x 90 mm:

Looking for a spacer I found silicone sheet (again 2 mm) and cut two spacer frames. Silicone has the positive side effect that it can be squeezed a little, so after placing it between the glass plates and giving it a gentle squeeze, the sandwich is held together by the vacuum between the parts:

You can actually lift it just by holding the upper plate. This was then sealed with silicone (this stuff is intended to seal stoves with, and can withstand 350 °C):

Cutting the glass was the hardest part of this.

As a finishing step the lid will get a frame made of U-shaped aluminium. The alumium has an inner width of 8.5 mm, so the 8 mm sandwich will fit nicely.