[QUOTEbut you need tempered glass or it will crack. ][/QUOTE]
No it won't.

Unless you've actually screwed the glass to the actual heating element. Pretty much any other method of attaching a glass plate to the aluminium one will leave more than sufficient wiggle room for the glass to expand.
I used bog standard cheapo photo fram glass with a couple of bulldog clips and it was fine.

Thumb screw wise - use four of the little nuts that came in the spares packet and print a set of these adjustment knobs.