Making tools from the tools you already own... This almost sounds incestuous - almost.
This is the next phase of our fairing construction for the University of Central Florida's 2007 ASME Human Powered Vehicle. Here we will take a positive male mold and begin making a female negative "tool." We call it a tool because it can be used to make many duplicate copies. This tool is in fact a negative female mold.
To reiterate, this whole project is daunting. However, each individual step is not too scary and within the capability of most people handy with a few simple power tools. On the other hand, the price tag is scary. For this half alone, we used 5 gallons of polyester resin (circa $130).
21 minute Video - Lots of Time Lapse (now with captioned instructions)
Your mold will be different - but here's the idea.
We want to make 2 female tools - one for each side. To do this, we must split the mold lengthwise (to allow easy mold release). So, the stand needs to hold the entire plug on it's side to present a "waterline" right down it's center. Positioning is critical as we don't want to make a mold that curves around itself more than 90 degrees (making removal impossible).
This stand is constructed using recycled 2X6's as a base and with 2x6 risers. The risers support an MDF board that will contain the plug cut out (recycled is better - these boards are expensive for what they are). The risers are a little more than 1/2 the width of the whole plug (that is, 1/2 the height of the plug when it's on it's side).
To make the cutout - place the plug on top of the stand. Then, use a square positioned against the plug and MDF board - mark the base position with a pencil. Repeat this over the entire plug without moving the plug. Once this is done, drill a hole and cut out with a jig saw.
Once you have your hole -- sand the edges down with a block so it won't be too rough on your plug - this is important for bondo application later. Test fit. Shave down. Test Fit and repeat until you have a good fit. You don't have to be 100% accurate - but you want a waterline that is flush with your MDF surface.
To make a cheap sanding block - use a scrap bit of wood and glue on sand paper - ha! you saved $5 on a sanding block that would likely just break after a few hours of use.