The extraction outlet on the back of the saw probably comes from a shroud around the blade - the one on my Scheppach table saw certainly does. The dust falling out of the base of the saw has bypassed that. I’ve built a similar cabinet to yours, but decided not to extract from the cavity under the saw, as I doubted that the extractor could generate enough airflow to pull the dust out. Instead, I just put a removable panel on the front, and clear it out occasionally with the shop vac.managed to get the saw on the cabinet. the sawdust was from cutting the wood for the cabinet even tho i had the shop vac on the back of the saw. hope the dust collection works as i hope!