all fiberglass or fiberglass / MDF?

I'm building a small sealed box for either an 8" or 10" sub that will hopefully fit in the small left corner of my 07 mazda 6 sport. Essentially, i'm going to be building a sub box with flat bottom, flat top and one flat side and then a a compound curve for the back of the sub. essentially, the shape will look like a a D on the floor.

I see three options.

1)I could make the flat pieces out of MDF, put rabbet edges on the interfaces between MDF and Fiberglass, and use some dowels to make sure the fabric keeps its shape on the curbed part.

2)Or i could frame it up using some sort of 1"x1" and then glass the whole thing.

3) i could build up the flat pieces in MDF, cut out the large expanses of MDF that aren't necessary (for additional volume) and then glass the whole box (similar to #2 but easier for fabrication purposes)

What are the advantages / disadvantages of each method? Which would be the easiest for someone who's only done fiberglass on boats?

Thanks in advance.

Fiberglass is weak in flat sections. IMO you should build as much out of MDF as possible. It will be cheaper as well.

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Fibreglass will allow you to get maximum box area as it would be moulded shape to the car.  MDF is chaeaper, its not as messy to deal with but it would invole alot more measuring and such depending on how closely you wanted the box to follow the contours of the car (which will affect airspace).  The comprimise is all flat sections and as much as u can in MDF, then glass any tricky bits

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Definately go the mdf route. You dont want to try and reinforce those flat sections with fiberglass its a pain trust me. Plus it saves so much time, frustration making all the flat areas out of mdf. Much easier.

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Learning the trade one fiberglass creation at a time!