If you're doing production, you do a female mold, and lay-up the glass. This is how most everything glass is made- from swimming pools, to a McMillan stock.
But making the mold is time consuming- and, of course, you have to actually make a pattern of the stock, to use as the male part to make the female mold...
When the glass is pulled from the mold and filler is used, it's got to have enough structural strength to be viable, yet "workable" to be routered/ milled for inletting.
There are multitudes of fillers that can be added to resins to achieve both the desired viscosity, "workability" and strength.
Very light fillers, like microballoons (glass bubbles) add a lot of volume, are very light, very easy to sand/fair- but have zero strength. Great for fairing, but that's it...
Others like high-density filler, or colloidal silica, add a lot of strength, but are heavy and difficult to sand.
There's a reason glass stocks are expensive- they're labor intensive, and if you price epoxy resins and fillers, you'll see they're quite expensive.
http://www.mcmfamily.com/mcmillan-stocks-faq.php