Have you worked with fiberglass before?
My father worked with fiberglass for many years, so I'm familiar with it but mostly from the sidelines. My own experience is with a few little patch jobs, so if anyone notices glaring errors below, jump in and put me in back in line.
You'll need:
1. Mould release. Several varieties, generally you'll spray it on the part you'll be making a mould for. Check with a local supplier for details.
2. Fiberglass cloth. Available in sheets, I think. Definitely in rolls.
3. Polyester resin or expoxy resin and the appropriate hardener (catalyst). Ask your local supplier for details.
4. (optional) Thick, honeycomb cardboard. This can be used to build up thickness.
4. Large scissors that your wife won't miss.
5. Electomechanical tubing (EMT) to add rigidity to the mould.
6. Paint brushes and, optionally roller brushes.
7. Gloves and other appropriate clothing.
8. A well ventilated work area. The resin fumes can get nasty.
Construction:
1. To make a parting line for the mould halves, you can cut long strips of thick cardboard, about 2 inches wide. Wrap tape around the strips to cover it so that resin won't soak into it. Tape the strip(s) along the edges of the part on the plane of the widest dimension. I hope you can follow that; I wish I had pictures.
2. Apply mould release on the side of the part you're starting on, including the parting line strip. Don't miss any surfaces. The mould release is to allow you to remove the mould from the part.
3. Pre-cut some strips of fiberglass cloth. Plan how/where you're going to apply them. In making a mould, you don't need to worry about strength in the same way you would on a finished part.
4. Cut one or more pieces of EMT either the full length of the part, or so two or more pieces can be overlapped to span the length. This will be used later to give the mould the stiffness to keep its shape.
5. Mix up resin with the correct amount of hardener. I would use a metal coffee can or similar. Don't mix up too much at a time. If you're a first-timer, start with maybe a quart or less 'til you get the hang of it. When the resin/hardener mix 'kicks', you could wind up with a warm can of useless, hardened resin.
6. Brush on a generous coating of resin to the part where you're about to put some glass. Just work on a smallish area at a time.
7. Lay a strip of fiberglass onto the wet resin, working it in to fully wet it with resin. Use a brush to add more resin on top, being sure to work out any air bubbles.
8. Brush resin onto an area adjacent to the piece you've placed. Apply the next strip of glass to it, but overlapping the first piece a fair amount.
9. Continue in this fashion until (a) you run out of resin mixture or (b) your resin 'kicks' (hardens) or (c) you get tired of it.
10. After covering the entire side of the piece and building up several layers, you can apply some strips of honeycomb cardboard. Cover both sides of it with resin and stick it on. Build up more layers of fiberglass atop this.
11. When you've built it up enough, place the EMT along the length of the piece and wrap thoroughly wetted (with resin) glass strips around the EMT in several places. At these places, lay more wetted strips over the EMT, adhereing it to the piece. You don't want the EMT to move and you want the mould stiff
12. When you're ready to repeat the process for the other half, remove the cardboard 'parting line' and apply masking tape along the lip of the finished parting line.
If you're new to this kind of thing, you might want to practice with some small parts first. I know I would.
