I used Garolite G10 for a base on my duct. There was a fair bit of under the seat fitting before I arrived at this shape. Meanwhile the RV box had been heavily modified and I think I can make this all work.

Pic 2 has the garolite G10 floxed in position and after cure a wide cut out. I also added some micro internally to transition the edges with the idea that this would help air flow.

Pic 3 is the modified box in place, which will get screwed into position. I need it removable for maintenance, or rather just in case it needs maintenance one day.

A flat garolite base

Exit cut out

Fitting the modified box