RE: Easy(ish) Combo for 150mph?
2oz at a minimum. I usually lay up the center sections of my wings with 3 layers of 2oz cloth, so I have pieces laying around to use for the firewall.
Quick method on this.....
Obtain four large, round, rubber balloons - 9" size or larger works well.
Blow it up once or twice - just to get it loose.
Sit down in a chair. Put the fuselage between your legs, firewall pointed up.
Take the inflated balloon - and push it over the firewall, quickly, as you let the air out - dont pop it.
(this is why you need 4, just in case)
The idea here is for the balloon to stretch over and collapse over the front of the fuselage, like a reverse vacuum bag.
Once you have that technique figured out, so the balloon does not come sliding back off....
Place the fiberglass cross in position .... make sure it fits.....
If you have the firewall pushrod hole, blindnuts and inserts installed (you should by this point) fill the holes with clay or wax.
Wet the cross down with epoxy (I suggest 30 minute type or finishing epoxy)
Position the cross on the firewall. Flatten the cross tabs against the fuselage sides, top and bottom.
Generally even out the epoxy - not a big deal - just get it close.
Then...... the moment of truth.....
Inflate the balloon - and force it down over the nose of the plane (as you practiced).
Let the epoxy cure over night ...
When you remove the balloon, you will have a nice, even, smooth surface, and an excellent bond.
Only requires a light sanding.
Then you can open up where the bolt holes and throttle pushrod locations are
As with Dave, I recommend epoxy for all of the glass work. PolyU is nice, but this is not for finish looks only - it is structural reinforcement, and the stiffness of glass/epoxy is beyond that of polyU/epoxy.