Here are some scarbs. They are from Reaper miniatures.

The only problem for warmaster is that the base does not fit 40x20 bases:

I use it for undead armylist with a profile like nurglins in daemon list, and with a special rule "it came from the sand" inspiring in the WHF rule.
I do it like this: undead general put a marker in the first turn of the battle in the place he wants the scarabs emerge. All the battlefiled is allow and both general could see where is it.
Every turn a dice roll is made to determine if the scarbs emerge. The possibility of emerge grows in every turn.
If the scarabs emerge, then a scatterd dice is rolle and a random distance is deterimne from the place asignated in the first turn. Also as scarab are alive they have initiative move.
The scarabs are no too powerfull, but count as enemies (-1 command) behind the lines. Also threaten artillery positions and gives some fun to the battle. It is particulary fun in siege games.
code 02492: Scarab Beetle Swarm
http://www.reapermini.com/OnlineStore/scarab%20swarm/sku-down/02492