Looking for niche ducting adapter

[jdb2]

I know a lot of HVAC experts read these forums so I'm hoping someone might be familiar with what I'm talking about. Specifically, I'm looking for a 280mm square to 120mm square/round ( doesn't matter ) telescoping light weight flexible reducing ( obviously ) ducting transition. ( Think giant square funnel ) I've searched everywhere and the closest I could find was this. Unfortunately its larger end is round -- i need square, but it almost has the right dimensions. ( the smaller end needs to be approx. 1/2 its current size )

Does anyone know where I might find such a beast?

Thanks,

jdb2

Edit : I forgot to mention that the item referenced in the link is not telescoping. ( Think of the bellows on an antique camera )

[jdb2]

The thing on the far right is almost exactly what I'm looking for, but I need something that is square.

jdb2