The procedure to make my moulds is as follows:
You put your pre-form into the sepeartion plane ( if necessary) and then you build these wooden boxes around the pre form. In my case I used plastic coated chipboard for the boxes. Everything is then covered sereveral times with a mould release wax. Then I apply the top coat, wich is a metal filled mould gelcoat (everything blue in the picture a remanents of the gelcoat). Then you let the mould gelcoat set. Next, one laminates about 3 mm of glassfiber into the mould box and fills it with what you called sand (which is pretty close). The sand actually is made of about 2 mm large glass spheres. It's a cheap product called Proaver http://www.poraver.com/en/. It easily mixes with resin, does not need to much of it and then solidifies to a block that can easily be sanded but can take the perssure of screw clamps. Finally you sand the proaver to a flat plane and seal the mould again with about 3 mm of glass fiber. This way you get a closed box wich is very stiff, easy to clean and will stay straight, evene when clamped together.