interior plastic modification help

[eaglzcirca33]

Hey fellas, I need some help in regards to the interior of my car. I have a 1970 Camaro I'm in the process of pro touring. The lower plastic interior pieces up front where the vents are located is what I want to modify. I would like to eliminate the vents and vent levers(I have new classic air). I want to turn the vents into speakers which will be easy but with the vent levers removed, it leaves holes that I want to eliminate(fill in). Does anyone have any suggestions on how to prep these pieces that will allow me to cover these holes up? I'm new with fiberglassing and plan on doing some fiberglassing custom work inside the car but does fiberglass stick to plastic or does anyone have any other suggestions? Thanks