Too bad (or maybe good) that you ordered a replacement quickly.
My camper has two really nice, bronze colored, double glazed, acrylic skylights with aluminum flanges (not plastic), thermal breaks, condensation gutters, and blackout pleated shades, mounted horizontally. A 1.5' x 2.5' in the bathroom and a 2' x 3' above the dinette.
The flat top pyramid design has 8" of clearance from the ceiling to the skylight inside surface. They bring lots of light into the camper, making it less of a dark cave.
They are made by
Dayliter. Mine are now 16 years old and still clear with no crazing, cracks or opaque areas.
They have
a model with an inside roof opening of 14.5" x 22.5" as shown in this PDF. The flange to flange edge dimension is bigger yet. This size would work depending on what else is near by on the roof.
It appears from the image you posted that the manufacturer didn't frame the skylight opening in your roof and are only relying on the plywood roof. I would be tempted to not cut the flange and offset the dayliter skylight, enlarge the opening as needed (peel back the EPDM roof and cut the plywood), and then frame in and trim the opening on the inside (screw in a piece of plywood to fill hole near the roof rack mount, glue in a piece of foam insulation, and cover the foam with a piece of the existing ceiling covering from the enlarged cutout scrap). The aluminum flange will seal the opening on the outside.
