That other pic that was posted is a good one. Wish I would have had that for a reference when I did my jars!
I did have one that cracked a little but it hadn't been thick enough I think. The others are fine.
As for bugs... they were sealed well and then outside in the pyramid and it was probably too cold. ( If they are sealed well they should be okay anyways.)
DAAS airdry clay is maybe what they suggested at michaels? I've used it before for other projects and it works quite well. I've also used Celluclay but it won't give a smooth finish without a lot of fiddling around. It is more a glorified paper mache. I prefer DAAS.
What about making it and then putting it on the top of the container after it is dry?
You wouldn't have the problem then of the glass preventing it from drying properly.