New Furnace Installation Question

Hi all,

I recently had a new furnace installed to replace an 18 year old furnace. The new units are smaller in dimension and a plenum was added to join to the existing ductwork above. The return air duct curved boot is sticking out at the back, and taking a closer look see that there is a decent gap and the rear of the boot is not screwed into anything. It also wouldn't reach the collar from the new furnace and from what I can see the collar at the front is against the boot and screwed together. What is typically done to close the gap? A new boot? aluminum tape? Something to adapt the back of the boot with the collar of the furnace? It is the same 16x25x1 filter size that the old furnace has, but the old furnace extended further in the back and was a different design where you could reach in from the furnace to the return air with the filter removed. The new furnace is sealed cabinet. I don't see any screw holes in the back of the return duct/boot, so it probably wasn't screwed into the collar of the old furnace.

Hopefully my explanation provides a visual.... any advice or comments?

Thanks in advance.

Edit: Added photo