have fiberglass in trunk corner ideas?

If you check this forum you will find plenty of people doing the same thing... There are plent of photo's and small how to's for it also. Anyways like you said I would make the base out of mdf. For the rest of it I would use fiberglass to create more room. That is also including glassing the front of the box and just creating a speaker ring out of mdf. If you glass the front then you can pull the ring out as far as you need to get the proper air space and depth so the woofer will fit.

Your best bet, I believe, Is to create the base then tape off the area you are planning to fiberglass (back, sides, and top) that are all carpet. Then once you get a good amount of masking tape I would tin foil it too and tape the tin foil down. Now that I am thinking about it I know for a fact that fiberglass doesnt stick well to the foil at all and definitely doesnt stick when hard, so You might want to double or even triple up on the masking tape so the resin wont go through it and just scratch the foil altogether. Or you could put masking tape, foil, and then more masking tape of the foil. (thats what I would do). Anyways I hope this helps. Good luck and dont forget to post pictures!!!