Vent Hole Lining

Curious what people recommend for lining the inside of vent holes? I have 3.5" diameter vent holes that go through insulation, so need some way of lining the hole to keep the insulation out and create a clean pathway between the inside of the sauna and the exterior. I see lots of duct options for 4" that look great but not so much for 3.5".

Any suggestions greatly appreciated.