Experience using mortar backgrounds in jar terrariums?

shot of failed adhesive attempt, pic included for more hits

I've seen mortar used in flat glass terrariums. My concern is cracking from something like expansion/contraction or subtle movements making contact with round and cheaper jar glass. Does anyone have personal experience with cements in jars?

I'm exploring the most affordable way to streamline a process of multiple one-offs for selling. What I need is full humidity tolerance and stiff enough body to set multiple small lava stones in place at once. Things that haven't worked: super glue/packing, silicone, foam, epoxy, several exterior rated adhesives.

Pic included of one that just crashed to the bottom. It's a bulbophylum orchid container meant to spread

and fill out long-term, so, it's constantly humid. This was an exterior all surface adhesive I already had, was clear dry but rewet and lost the bond in humidity.