Check out Daniel Spurr's "Upgrading the Cruising Sailboat". He has instructions on adding a seahood fabricated out of fiberglass that can be expanded in size for your need (I think).
You're basically building an inverted dinghy if you think about it. I'd make a female mold out of plywood with outside framing (larger by whatever thinkness you want overall). Fair up the inside well and round all the seams with a cheap filler. Then wax it heavy and build it up with layers of glass and matt until you get it to the strength you need. You can glass in foam "beams" - the strength is from the rigid fiberglass tube over the form so the center doesn't need to be strong.
If you make it removable it will be appreciated down the road if you ever need to re-engine or pull the old for rebuilding.
Charlie P. P31-2 #80 CATNAP
To travel hopefully is a better thing than to arrive - R.L. Stevenson
I suspect that, if you should go to the end of the world, you would find somebody there going farther . . . - H.D. Thoreau