Very little. Think of it this way: Every RV ever made has a roof vent in it. Most have a roof air unit as well, a few holes for fridge vent, plumbing vent, TV antenna, stove/oven vent, skylight in the bathroom, etc. Fiberglass is easy and fun, and easy to repair a mistake.
I'm no expert but have done 5 or 6 of these. I have done in them in metal roofs as well with no problems but still I think you'd be unhappy with the wind noise putting one between the seats.
I can think of 10 or more folks here that have put one in their PH top and I've never heard a complaint.
Batteries not included
Must be present to win
Tax, tag, title not included