I'm looking to improve the ventilation to a suspended timber floor with
some new vents - they'll open to an outside path that has built up over
the years to near internal floor level.

I see there's these 'periscope' type things:

https://manthorpebuildingproducts.co.uk/product/g965-dual-extended-underfloor-vent

But it seems to me they'd take a fair chunk out of the 9" solid brick
walls, and the air flow doesn't look that good.

Would flat letterbox-sized grilles over a say 10" deep 'pit' feeding a
horizontal cut through the wall below the existing floor work?

I can see the issue of debris falling in, but if the grilles are
reasonably removable I can't see that being a dire problem. If it is
doable, is there a source of grille/conduit - can't seem to see anything
online.

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Cheers, Rob