6" Ducting straight to in-wall rectangular ducting
I'd just get a length of rectangular 3-1/4 x 10 duct, capped on both ends, and connect to it low and high with starting collars. Seal things up nicely with silicone caulk or foil tape.





                           _____  
                         ||     || &lt;-- end cap
                        __|     |
                          .     |
starting collar in --&gt;    .     | &lt;-- box duct
                        __.     |
                          |     |
                          |     |
                          |     |
                          |     |__
                          |     .
                          |     .   &lt;-- starting collar out
                          |     .__
                          |     |
                         ||_____||