I've done some composite work and make the odd flight case for people using sheets of a carbon/kevlar/glass mix - the easiest way I have found of getting a glassy smooth finish is to use vacuum infusion onto a rigid sheet of perspex "mould" on a flat surface - you get glassy smooth finish with no bubbles.

Not sure, but you might be able to pull a thin infusion type resin through large sized grains of "rock" of some kind mixed with chopped fibres (I keep all my carbon/kevlar/glass off cuts for this purpose). If you can't vacuum through the aggregate, then pouring into a box mould with a perspex bottom should achieve good results. All the bubbles and crappy surface will be on the top and the shiny flat surface on the bottom, which you can then demould and flip over.

Heat would be my only concern, as large volumes which don't have a matrix of some kind tend to go off with cracks in them, but a few sheets of glass or carbon layered in the mixture should help solve that.

(Eyes up the gallon container of resin on the other side of the cave ;-) ).

Edit - nearly forgot - this link might hel with ratios http://www.talkcomposites.com/11/Res...ge-calculation