Would a hard headliner fit back into the civic?

Okay, I have nearly all of my interior out of the 93 hatch while I'm getting the car ready to be painted(Tuesday I hope!), and trying to get some ideas for the interior.
My first problem that I wanted to start with was the headliner. It looks like crap(it was dyed and parts of it is peeling a little even though most of it looks brand new), and was wanting to make a fiberglass one. I was just gong to lay around 2 layers of fiberglass on top of the headliner, using the headliner as a mold, to make a near exact match of the other headliner. After I get the fiberglass off of the old headliner, I would sand it down, maybe add an extra layer of fiberglass to give more sanding room, and put body filler on it to make it perfectly smooth, and then paint it.
But would it fit back into the hatch after it's hard like that? I have some of the flex additive that you put in your paint to paint bumpers that will flex a little, so would that maybe help some?
I also like this idea, because it will give me a GREAT place for my 6x9 speakers that I've been meaning to get put in the back of the car somewhere. I can just make a small enclosure in the middle of the headliner closer to the back, for 2 6x9 speakers.
My first problem that I wanted to start with was the headliner. It looks like crap(it was dyed and parts of it is peeling a little even though most of it looks brand new), and was wanting to make a fiberglass one. I was just gong to lay around 2 layers of fiberglass on top of the headliner, using the headliner as a mold, to make a near exact match of the other headliner. After I get the fiberglass off of the old headliner, I would sand it down, maybe add an extra layer of fiberglass to give more sanding room, and put body filler on it to make it perfectly smooth, and then paint it.
But would it fit back into the hatch after it's hard like that? I have some of the flex additive that you put in your paint to paint bumpers that will flex a little, so would that maybe help some?
I also like this idea, because it will give me a GREAT place for my 6x9 speakers that I've been meaning to get put in the back of the car somewhere. I can just make a small enclosure in the middle of the headliner closer to the back, for 2 6x9 speakers.