Simple adapter that fits snugly over this wall vent from Amazon:
Spigot fits into 5" (125mm) ducting to allow use of a hose clip, cable tie etc.
Print with something suitable for higher temps, but most materials probably fine. I printed with a 0.8mm nozzle to speed things up and tuned the wall thickness to print nicely with this size.
Even though you can find many cheap adapters online, they all seem to be slightly different sizes and some require lengths of duct to mate, so I ended up printing this. Works well for the vent listed, but YMMV.
The author marked this model as their own original creation.