Hey, I was thinking of something from my days of building racing sailboats.
If you want to make the shell lighter and stiffer, you could try this:
Make the molds for the shell as negative molds (like bathtubs)
When you build up the layers, put in a layer of something like this:
Klegecell
and then use a vacuum pump to apply pressure. You do this after your outer couple of layers have been applied and have set.
To do this, after the outer shell is set up, you have to put in all the following and get the vac pump going before the resin starts to set.
1- a layer of mat or cloth
2- apply resin, roll out the bubbles
3- lay in pieces of klegecell (they won't fit perfectly - that is what the vacuum is for.)
4- lay a big sheet of plastic in the mold. Seal its edges to the edges of the mold, except...
5- in one spot, put the hose from a vacuum pump under the plastic sheet, and seal all around the hose.
6- turn on the pump.
7- watch as the plastic sheet is sucked down on to the surface of the shell, klegecell, and mat
8- smile a satisfied smile as you look for leaks
9- let it run until the resin is fully set, probably overnight
Different stuff to note:
The adhesive that you use to seal the edges needs to be pretty good. We used some kind of 3M product that we called "monkey poop". Helpful, I know. It was kind of like high-tech modeling clay. It came on rolls with a wax paper backing. It did not dry out and was super sticky.
Nothing sticks to wet resin, so when you apply the layer of mat and resin, be careful not to get it on the edges of the mold.
The vacuum pump hose may ingest resin, so think about a trap of some kind, or use hose that you don't care too much about.
Put a little scrap of klegecell in front of the end of the vacuum hose to keep from sucking the plastic sheet in.
Of course, experiment with it first since it is a little hectic trying to get everything in the mold and sealed up before the resin kicks.
The advantage of this method is that even if you only get something like 36"H2O vacuum, you are applying 190 lbs of force per square foot, which gets everything nice and smashed together.
Here's a video:
Vacuum Bagging video
The klegecell will make the structure stiffer and is a way to give the shell some three-dimensionality to reduce flex, oil-canning, and all that. Plus, it's fun to say "Klegecell" and "vacuum bagging"
It occurs to me now that you guys probably already knew all this, but in case you didn't, I hope it was helpful.
The other two were cut to fit and attached. The nice thing about PETG is that you can fold the sheet in half, it it takes the bend 
Linear Mode
