I tried for an hour, in the attic, to compress or expand the solid duct pipe to fit inside or outside the vent opening...until the adjustable elbow I was using broke.

The uncrimped side of the solid duct pipe is significantly larger than the vent opening, so I feel like increasing the crimps and managing to squeeze it in would leave it under some tension.

I'll look for some kind of adapter and, barring that, I think my best bet will be to find some way to smooth out the crimps in the duct slightly and have it fit over the vent opening.

Second, the front portion below the vent opening should have weep holes so that any moisture or condensation could escape