I have read most of the posts in this thread with interest. If I might add a couple of observations.

When making fibreglass parts the strongest stiffest part has the least amount of resin with adequate wet-out. That is why the vacuum bag is a popular technique. A quick explaination for those not familiar with the process. Glass lay up on mold with resin worked in. Then a BARRIER film which has very small holes in it. Then sometimes a layer of coremat to absorb excess resin. Then a nonporus plastic bag is sealed to the edge of the mold Area. ( or the whole mold can be placed into a bag and sealed) What this does is use atomospheric pressure to force the resin/glass together while pulling the excess resin through the very small holes in the BARRIER film. When cured the barrier film acts as a mold release between the fiberglass part and the excess cured resin, which can be peeled away.

What if you used the same process on the open side of the mold. This would compact the mixture into the mold with a force of 2000 psf, while allowing the excess epoxy / air to come out. Just pulling a vacuum above the mixture surface ( as in degassing ) only expands the air bubbles to make them "lighter " in relation to the rest of the resin and / or mixture so they try to float to the top. But that only works when the mix is liquid so the bubbles can move through it. Because the bag is flexible and can move down onto the casting it applies a 14.7 psi pressure ( approx 2000 psf ) to the whole casting open face. On a 4' x 4' casting that is almost 17 tons! If you are worried, you can always use a lower vacuum.
Vibration tries to get the mixture to settle and help the aggregate to "fit" into each other, which it good but the only pressure is the weight of the casting itself. It seems to me that a combination of the two techniques would give the highest density casting with the least air entrained.

CKELLOUG
If you put random carbon fiber into the bottom surface of the part it doesn't have anywhere near the effect of continuous fibers running 90 degrees to the bending monent, that is why we use cloth instead of mat. I believe you can get carbon cloth with a majority of the fibres running in one direction.

Without trying to hijack the thread, one last observation. While a rigid, vibration damping base might be great, the weakest design element I have seen in most of the home built gantry machines are the uprights for the bridge side. They are much too weak and flexible. If you think about the cutting forces involved.
When you plunge the force is up which puts the gantry sides into tension and tries to bend the crossbeam. This is OK since the Z depth will go to where it belongs as the Z stops going down and the beam straightens. Most of the cutting forces are generated when cutting in the XY plane. The resultant force is 90 degrees and opposite to the cutting edges at any given instant. This puts all the force into flexing the gantry side. And since the gantry sides are normally longer along the direction of travel there is a constantly variable force shaking the gantry. That is why if you look at any well made commercial router, the supports are massive square tubes.

Regards,
Will