convertible tonneau cover

So I have been toying around with the idea of making a tonneau cover for the back seat of my convertible for about 2 years now. I have decided to make it a reality. The problem is that I am the biggest noob on the planet when it comes to this fabricating stuff. I have read various posts and threads on this board as recommended by my friends who have used this information to build their own projects. Honestly I have learned very little because I cannot find the basic information anywhere. I thought it would be as simple as building the mold and then laying the fibreglass over the mold, shaping it as you go, and then preparing it for paint. Obviously there are a ton of steps in between so I have a bunch of questions to ask before I start. 1- is it necessary to make both a male and female part to the mold to make an object from Fiberglass? 2- I have noticed that when making any type of cavity cover that there is always an MDF portion that is left inside the object. Is this necessary or can the fiberglass be built up enough to support itself? 3- If I were to use styrofoam insulation to shape the mold would it dissolve when I lay the fiberglass on it? Or is this an instance where using tinfoil is the solution? Here is my plan of attack ....please give input as necessary to correct me so that I get this right the first time. I drive a 99 Mitsubishi Eclipes Spyder. I want the rear seat cover to have 2 humps (see the Solstice/Sky style trunk.....Ferrari type styling) My plan was to lay a piece of plywood over the rear seat covering the entire area. Next I plan to attach the sono tubes cut to the correct angle to give me the desired shape. I was then going to use the styrofoam insulation on either side of the tubes to shape the cover for aerodynamics. This is pretty much where my plans end at the moment as I have no idea what should be next. My original plan was to cover the mold with fleece at this point and then start laying the fiberglass over the mold. Is it possible to still get the desired shape now or will I need a female side to the mold to create the proper form? I really hope I am not annoying anyone with this series of question.. but I want to get it as close to right the first time... as humanly possible. No one wants to toss good money over bad making too many attempts of getting so frustrated that I end up quitting the project. Any help/suggestions that can be offered will be greatly appreciated. Thank you in advance for anything you can suggest.

I'm not much help in this area but ill say what i can. first you only need one half of the mold, and i suggest doing it without a mold to begin with. my plan of attack would be; lay the ply wood base over the back seats. cut a series of ribs from 3mm mdf board, this would form the shape u want. next i would stretch fleece over the part. cover the fleece in 2 coats of resin. turn the part over, then jigsaw or cut away the base board as well as possible, remove the ribs through the new hole. lay 3 layers of fiberglass through the hole onto the fleece, turn the part back over lightly sand the fleece cover. apply a thin bog layer over the part which should have remained practically smooth. sand bog and then sand heaps more to prep for paint or vinyl. this means you have no mold which reduces complexity and time and cost. unless u want to make multiples. polyester resins will melt polystyrene, they are both from the same chemical family poly-poly so it acts like mineral turps and oil based paint or water and water based paints. there are fiberglass out there which wont melt polyester i think they are acrylics or something honestly no idea, i have used them once and they small horrid in my opinion. u can remove all the MDF people leave it for a variety of reasons mainly ease when making a simple once off part. fiberglass is incredibly strong, gram per gram stronger then steel so yes it can hold its own shape, there is an art to doing it though. without going into the mechanics too much basically you can imagine a force eg a weight on the middle of the plank, the direction the force acts is the same direction that the material supports it. so a thin 1mm plank bends cause when the weight sits on it, it only has 1mm of material supporting it, if that plank it 10cm wide and u turn it on its side, with the same weight it no longer bends because there is 10cm of material stopping it bending. that's a simple version of what u want to achieve. on your cover try incorporate either a lip that run around the whole part or some vertical sections of fiberglass to make the part strong, a flat board(not that u envisioned it) will bend. i find it easiest to imagine the whole thing is made out of 1mm steel sheet will the sheet bend and warp if i sat on it and how can i change it. hope that helps and isnt more confusing

that information certainly makes me feel a bit better about my task. The frame now becomes easier to construct if I do not need to make a full mold. I know now not to use the insulation as a contruction material. And I have a new plan of attack which looks fairly easy. Just one question... what is a "bog layer" I understood pretty much everything else that you mentioned.

i use a product called builders bog which is a fiberglass putty basically, used to straighten car bodies or patch wood work ect. i think in the states the common one is called rage gold. the sort of product is mentioned a number of times here i don't have any links off the top of my head unfortunately. the idea is to fill any diverts and smooth the surface.