Crawl space Leca - ventilation without duct?

[Alko]

Hello! The company that built the leca foundation (8m2) for our extension did not leave holes for ventilation (because I told them I would have vents in the hatches which I later changed), so I thought about drilling/sawing my own and installing vents.

But the vent grilles I want to use are square, and I can't find any square ducts.

I'm wondering if I really need a duct, or if I could just plaster the inside with a hard cement mortar?

The vents are of the type that are plastered in:

 

[Liteavvarje]

Not common with pipes through foundation walls, just a hole with a vent on the outside

 

[Claes Sörmland]

Exactly, plaster

Considering if I really need a duct, or if maybe I can just plaster the inside with a hard cement mortar?
[image]

That's how it usually looks!

 

[Alko]

Sounds good, thanks for the reply!