Homemade periscopes for under floor insulation

Any tips for making your own ventilation periscope for underfloor insulation? A friend is insulating under his floor and needs to put in periscopes as the air vent is only just below floor level. Off-the-shelf periscopes aren’t going to work because the air vent is very large and the solid stone wall is very thick (600mm). I remember Nick Parsons did a webinar in which he mentioned making periscopes out of Kingspan or similar. Just want to check how that works (condensation risk)? And if anyone has any pictures of their own efforts?