I don't top layer my soils. Once the plants have been purchased the plants are given a steam wash, old soil removed and a new plant pot given with fresh new compost. I use the Westland bonsai compost as it works really work for ficuses and has no perlite, just mulch.
I've had no problems so far and the top layer seems to dry off from the exposed heat. I would suggest doing the same otherwise the soil might not have enough air to properly dry out, becoming too humid underneath the top layer which can lead to root rot of the plants.
I own both sexes of veiled chameleon. None of them have tried to eat the soil.
As Carlton has already suggested you can try medium sized decorative stones
If you are worried about crickets breeding in it unfortunately you can't avoid that. Insect mesh is too wide for the ovipositor and would still find a way to breed.